...this seems like an easy question but i've learned to never say never.
..is the rear seat in a '71 Valiant 4 door the same as a 2 door rear seat.
..and is it the same as a scamp.Duster Dart rear seat 2 and 4 door?

From what I've seen dealing with mostly 73+ A-bodys...

4 door lower seat cushions have a pronounced curved leading edge (presumably to have more room for entry).

2 door lower cushions have a straight leading edge (no entry room needed cause they butt into the rear door panel).

I also tried to put an upper cushion from a 4 door '74 Dart in my '76 Duster...no go.