Are rear seats interchangable on all A-bodies?

This topic has me curious.
I've messed around with 67 and later A body cars and have only noticed that the bottom sections are different. Maybe I have been just lucky but I thought that every upper section has appeared to be the same in terms of size and fitment....2 door, 4 door, Sedan or Duster/Dart Sport. Have I just somehow got lucky?