The inner door frame is totally different on a 68 door. You really have to have them side by side to see the difference. You can make a 70 door work on a 68 but you can't make a 68 door work on a 70. Well you can but you would have to use 68 door panel's. The door lock rod is totally different on a 70 door.
69 inner door made to work on my 68 nova. Same as 70.
68 nova inner door frame.
Also 68 window cranks are located about one and a half inch farther forward than a 69-70 nova door frame, this is the picture of the 68 door frame.
So if the window cranks are in a different location on a 68 door you guessed it the window regulator for a 68 is longer than a 69-70.