I did the same thing with my '66 Cuda. I picked up a '68 Dart GT that had a trashed body, but solid running gear. It had a 727 instead of a 4spd, but the same deal applies.
I took the engine, trans, axle and driveshaft from the '68 and it was a straight bolt in on the '66. I even swaped the torsion bars which were a little bigger and the springs which had an extra leaf. Figured it would give a stiffer, more stabe ride.
I think the difference in the wheelbase is all in the more forward position of the K-frame and front suspension on the '67 and later cars. It looked to me like they were dimensionally the same from the motor mounts back. Could be a little different I guess, but not enough to be a problem. It's possible the driveshaft maybe sat a hair further in past the rear tranny seal.
I'm not a Mopar expert, but I'm assuming this forward position is why all the headers and other aftermarket goodies that are specific to '67 and later cars wont work on the early A's. I know I wanted to try and mount up the '68 sway bar and there was no way that was happening.
