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First about that comma:-
A - "Truly I say unto you, .................. Today you will be with me in paradise" or
B - "Truly I say unto you today, ........ You will be with me in paradise".
Two reasons:-
First: If B above is true, what purpose does the word "Today" serve? It is not necessary, a superfluous addition. If this is what Our Lord had meant it would have been easier (and better) for Him to have said "Truly I say unto you, You will be with me in paradise", leaving out the "today" altogether. If B is correct the word "today" adds absolutely nothing (except the possiblility of misinterpretation and confusion). In fact, in view of the second reason given below it is utterly misleading.
Second:
Look up all the other places in Matthew, Mark, Luke and John where Our Lord says "Verily I say unto you", "Verily I say unto thee" or "Verily, verily, I say unto you" or "Verily, verily, I say unto thee" (all KJV).
In all, these phrase types are said no less than 77 times. It was Our Lord's way of drawing attention what he was going to say next, often because it was going to be a surprising truth. You are right there were no commas in the original but in view of this phrase being given 76 times I think we are entitled to believe He used exactly the same phrase in Luke 23:43 and for the same reason.
Read through all the following uses of those phrase types (as given in English by KJV) and I think you will find it impossible to come to any other conclusion:-
From Matthew's Gospel:-
5:18,26; 6:2,5,16; 8:10; 10:15,23,42; 11;11; 13:17; 16:28; 17:20; 18:3,13,18; 19:23,28; 21:21,31; 23:36; 24:2,34,47; 25:12,40,45; 26:13,21,34
From Mark's Gospel:-
3:28; 6:11; 8:12; 9:1,41; 10:15,29; 11:23; 12:43; 13:30; 14:9,18,25,30
From Luke's Gospel:-
4:24; 11:51; 12:27; 13:35; 18:17,29; 21:32; and the one under debate Luke 23:43
From John's Gospel:-
1:51; 3:3,5,11; 5:19,24,25; 6:26,32,47,53; 8:34,51,58; 10:1,7; 12:24; 13:16,20,21,38; 14:12; 16:20,23; 21:18
In each case the phrase is simply "Verily I say unto you" or a variant without the superfluous addition of the word "today" in the phrase... unless Luke 23:43 is a peculiar exception, but why should it be, and what purpose does the word "today" in Luke 23:43 then serve?
Regards