If the holes for the weatherstripping are the same, just change everything to a '68 and it will work. The 4 doors had wing windows through all of the years as i recall, so the barrier here is the weatherstripping. Take the weatherstripping off the car, fit the '68 doors and see what you need to make it seal.
Your car would look better with the '68 doors and the wing windows, IMHO, and it shouldn't be that hard to do.
BTW, Dodge made Darts and Plymouth made Valiants/Scamps.
The doors that are really different in those years are the 2 door posts which had a upper door frame and would not fit at all, similar to a 4 door upper frame just extended.
I hope this helps, some pics of the doors that you want to install would help a lot.
Good luck on your project and post some pics when done.
Tom.