63-66 seat should interchange. 4-door rear seat cushions are more rounded on corner to get feet past them. The 2-door rear seat cushions have more square corners.
64-66 used the same door handles and arm rests. Door panels ought to interchange as well. If you're a bit of a DIY person, the door panels are easy to make so you could do some custom work. Though the holes in the door panel are die cut, once you have the covering on, no one will be the wiser.
The chassis changed in 67 which may have resulted in changes to seat width and mount locations. I'll be happy to take measurements on my 73 Dart Custom. It would be neat if you could put a pull down center arm rest front bench in your 64 Valiant, it that's the way you want to go.
Sadly, no A-bodies were made with rear pull down center arm rests.
The Dart and Valiant headliners from 63-66 will not interchange due to different roof length and C-pillar width, except in the wagons.
