One does not need a doctorate in Greek to understand the original had very sparse punctuation.
"All of the Greek New Testament originals were written in ALL CAPITAL LETTERS with no spaces and probably no punctuation, and all of the earlier manuscripts are in this style, whether on parchment or papyrus. This is because the Greek alphabet did not have punctuation until at least the II century, and there were no minscule (lower case) letters until much later."
from The Transmission of the Greek New Testament.
So, when Luke 23:43 is considered with John 20:17 and many other N.T. scriptures such as...
1Co 15:23 But every man in his own order: Christ the firstfruits; afterward they that are Christ's at his coming.
It becomes apparent that the text should be read as...
Luk 23:43 And Jesus said unto him, Verily I say unto thee today, shalt thou be with me in paradise.
While it is true that the originals had no punctuation and "Verily I say unto thee today, shalt thou be with me in paradise" is a possible rendering, it is unlikely to be correct in my opinion because it is inconsistent with the way that Jesus talked. Jesus says "Verily I say unto you" over seventy times in the four Gospels combined and not once does he say "Verily I say unto thee today,".
Mat_5:18
For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
Mat_5:26
Verily I say unto thee, Thou shalt by no means come out thence, till thou hast paid the uttermost farthing.
Mat_6:2 Therefore when thou doest thine alms, do not sound a trumpet before thee, as the hypocrites do in the synagogues and in the streets, that they may have glory of men.
Verily I say unto you, They have their reward.
Mat_6:5 And when thou prayest, thou shalt not be as the hypocrites are: for they love to pray standing in the synagogues and in the corners of the streets, that they may be seen of men.
Verily I say unto you, They have their reward.
Mat_6:16 Moreover when ye fast, be not, as the hypocrites, of a sad countenance: for they disfigure their faces, that they may appear unto men to fast.
Verily I say unto you, They have their reward.
Mat_8:10 When Jesus heard it, he marvelled, and said to them that followed,
Verily I say unto you, I have not found so great faith, no, not in Israel.
Mat_10:15
Verily I say unto you, It shall be more tolerable for the land of Sodom and Gomorrha in the day of judgment, than for that city.
Mat_10:23 But when they persecute you in this city, flee ye into another:
for verily I say unto you, Ye shall not have gone over the cities of Israel, till the Son of man be come.
Mat_10:42 And whosoever shall give to drink unto one of these little ones a cup of cold water only in the name of a disciple,
verily I say unto you, he shall in no wise lose his reward.
Mat_11:11
Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he.
Mat_13:17
For verily I say unto you, That many prophets and righteous men have desired to see those things which ye see, and have not seen them; and to hear those things which ye hear, and have not heard them.
Mat_16:28
Verily I say unto you, There be some standing here, which shall not taste of death, till they see the Son of man coming in his kingdom.
Mat_17:20 And Jesus said unto them, Because of your unbelief:
for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you.
Mat_18:3 And said,
Verily I say unto you, Except ye be converted, and become as little children, ye shall not enter into the kingdom of heaven.
Mat_18:13 And if so be that he find it,
verily I say unto you, he rejoiceth more of that sheep, than of the ninety and nine which went not astray.
Mat_18:18
Verily I say unto you, Whatsoever ye shall bind on earth shall be bound in heaven: and whatsoever ye shall loose on earth shall be loosed in heaven.
Mat_19:23 Then said Jesus unto his disciples,
Verily I say unto you, That a rich man shall hardly enter into the kingdom of heaven.
Mat_19:28 And Jesus said unto them,
Verily I say unto you, That ye which have followed me, in the regeneration when the Son of man shall sit in the throne of his glory, ye also shall sit upon twelve thrones, judging the twelve tribes of Israel.
Mat_21:21 Jesus answered and said unto them,
Verily I say unto you, If ye have faith, and doubt not, ye shall not only do this which is done to the fig tree, but also if ye shall say unto this mountain, Be thou removed, and be thou cast into the sea; it shall be done.
Mat_21:31 Whether of them twain did the will of his father? They say unto him, The first. Jesus saith unto them,
Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you.
Mat_23:36
Verily I say unto you, All these things shall come upon this generation.
Mat_24:2 And Jesus said unto them, See ye not all these things?
verily I say unto you, There shall not be left here one stone upon another, that shall not be thrown down.
Mat_24:34
Verily I say unto you, This generation shall not pass, till all these things be fulfilled.
Mat_24:47
Verily I say unto you, That he shall make him ruler over all his goods.
Mat_25:12 But he answered and said,
Verily I say unto you, I know you not.
Mat_25:40 And the King shall answer and say unto them,
Verily I say unto you, Inasmuch as ye have done it unto one of the least of these my brethren, ye have done it unto me.
Mat_25:45 Then shall he answer them, saying,
Verily I say unto you, Inasmuch as ye did it not to one of the least of these, ye did it not to me.
Mat_26:13
Verily I say unto you, Wheresoever this gospel shall be preached in the whole world, there shall also this, that this woman hath done, be told for a memorial of her.
Mat_26:21 And as they did eat, he said,
Verily I say unto you, that one of you shall betray me.
Mat_26:34 Jesus said unto him,
Verily I say unto thee, That this night, before the cock crow, thou shalt deny me thrice.
Mar_3:28
Verily I say unto you, All sins shall be forgiven unto the sons of men, and blasphemies wherewith soever they shall blaspheme:
Mar_6:11 And whosoever shall not receive you, nor hear you, when ye depart thence, shake off the dust under your feet for a testimony against them.
Verily I say unto you, It shall be more tolerable for Sodom and Gomorrha in the day of judgment, than for that city.
Mar_8:12 And he sighed deeply in his spirit, and saith, Why doth this generation seek after a sign?
verily I say unto you, There shall no sign be given unto this generation.
Mar_9:1 And he said unto them,
Verily I say unto you, That there be some of them that stand here, which shall not taste of death, till they have seen the kingdom of God come with power.
Mar_9:41 For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ,
verily I say unto you, he shall not lose his reward.
Mar_10:15
Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child, he shall not enter therein.
Mar_10:29 And Jesus answered and said,
Verily I say unto you, There is no man that hath left house, or brethren, or sisters, or father, or mother, or wife, or children, or lands, for my sake, and the gospel's,
Mar_11:23
For verily I say unto you, That whosoever shall say unto this mountain, Be thou removed, and be thou cast into the sea; and shall not doubt in his heart, but shall believe that those things which he saith shall come to pass; he shall have whatsoever he saith.
Mar_12:43 And he called unto him his disciples, and saith unto them,
Verily I say unto you, That this poor widow hath cast more in, than all they which have cast into the treasury:
Mar_13:30
Verily I say unto you, that this generation shall not pass, till all these things be done.
Mar_14:9
Verily I say unto you, Wheresoever this gospel shall be preached throughout the whole world, this also that she hath done shall be spoken of for a memorial of her.
Mar_14:18 And as they sat and did eat, Jesus said,
Verily I say unto you, One of you which eateth with me shall betray me.
Mar_14:25
Verily I say unto you, I will drink no more of the fruit of the vine, until that day that I drink it new in the kingdom of God.
Mar_14:30 And Jesus saith unto him,
Verily I say unto thee, That this day, even in this night, before the cock crow twice, thou shalt deny me thrice.
Luk_4:24 And he said,
Verily I say unto you, No prophet is accepted in his own country.
Luk_11:51 From the blood of Abel unto the blood of Zacharias, which perished between the altar and the temple:
verily I say unto you, It shall be required of this generation.
Luk_12:37 Blessed are those servants, whom the lord when he cometh shall find watching:
verily I say unto you, that he shall gird himself, and make them to sit down to meat, and will come forth and serve them.
Luk_13:35 Behold, your house is left unto you desolate: and
verily I say unto you, Ye shall not see me, until the time come when ye shall say, Blessed is he that cometh in the name of the Lord.
Luk_18:17
Verily I say unto you, Whosoever shall not receive the kingdom of God as a little child shall in no wise enter therein.
Luk_18:29 And he said unto them,
Verily I say unto you, There is no man that hath left house, or parents, or brethren, or wife, or children, for the kingdom of God's sake,
Luk_21:32
Verily I say unto you, This generation shall not pass away, till all be fulfilled.
Luk_23:43
And Jesus said unto him, Verily I say unto thee, To day shalt thou be with me in paradise.
Joh_1:51 And he saith unto him,
Verily, verily, I say unto you, Hereafter ye shall see heaven open, and the angels of God ascending and descending upon the Son of man.
Joh_3:3 Jesus answered and said unto him,
Verily, verily, I say unto thee, Except a man be born again, he cannot see the kingdom of God.
Joh_3:5 Jesus answered,
Verily, verily, I say unto thee, Except a man be born of water and of the Spirit, he cannot enter into the kingdom of God.
Joh_3:11
Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our witness.
Joh_5:19 Then answered Jesus and said unto them,
Verily, verily, I say unto you, The Son can do nothing of himself, but what he seeth the Father do: for what things soever he doeth, these also doeth the Son likewise.
Joh_5:24
Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life.
Joh_5:25
Verily, verily, I say unto you, The hour is coming, and now is, when the dead shall hear the voice of the Son of God: and they that hear shall live.
Joh_6:26 Jesus answered them and said,
Verily, verily, I say unto you, Ye seek me, not because ye saw the miracles, but because ye did eat of the loaves, and were filled.
Joh_6:32 Then Jesus said unto them,
Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven.
Joh_6:47
Verily, verily, I say unto you, He that believeth on me hath everlasting life.
Joh_6:53 Then Jesus said unto them,
Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you.
Joh_8:34 Jesus answered them,
Verily, verily, I say unto you, Whosoever committeth sin is the servant of sin.
Joh_8:51
Verily, verily, I say unto you, If a man keep my saying, he shall never see death.
Joh_8:58 Jesus said unto them,
Verily, verily, I say unto you, Before Abraham was, I am.
Joh_10:1
Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber.
Joh_10:7 Then said Jesus unto them again,
Verily, verily, I say unto you, I am the door of the sheep.
Joh_12:24
Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit.
Joh_13:16
Verily, verily, I say unto you, The servant is not greater than his lord; neither he that is sent greater than he that sent him.
Joh_13:20
Verily, verily, I say unto you, He that receiveth whomsoever I send receiveth me; and he that receiveth me receiveth him that sent me.
Joh_13:21 When Jesus had thus said, he was troubled in spirit, and testified, and said,
Verily, verily, I say unto you, that one of you shall betray me.
Joh_13:38 Jesus answered him, Wilt thou lay down thy life for my sake?
Verily, verily, I say unto thee, The cock shall not crow, till thou hast denied me thrice.
Joh_14:12
Verily, verily, I say unto you, He that believeth on me, the works that I do shall he do also; and greater works than these shall he do; because I go unto my Father.
Joh_16:20
Verily, verily, I say unto you, That ye shall weep and lament, but the world shall rejoice: and ye shall be sorrowful, but your sorrow shall be turned into joy.
Joh_16:23 And in that day ye shall ask me nothing.
Verily, verily, I say unto you, Whatsoever ye shall ask the Father in my name, he will give it you.
Joh_21:18
Verily, verily, I say unto thee, When thou wast young, thou girdedst thyself, and walkedst whither thou wouldest: but when thou shalt be old, thou shalt stretch forth thy hands, and another shall gird thee, and carry thee whither thou wouldest not.
So I believe the consistency in which Jesus spoke is overwhelmingly against placing the comma after "today".