I don't want to sound like a schmuck getting on these boards and trying to sell something, but we have a custom seat that is a direct bolt in replacement (no modifications) for 55-57 Chevy's. It's a split back bench seat, comes with powdercoated frame, foam, recliners, mounting risers and hardware, and sliders.
The biggest problem with using a seat from a later vehicle is the distance it sits off the floor. Try something from a current "big car" like a full size Chevy or Buick or Olds. Just hit the boneyards and have your tape measure in hand.
One of may friends put a 60/40 set from a mid eighties Olds midsize directly into a 36 Chevy 2dr Sedan (a little bitty car inside)). He just cut out the armrest from the 60 side, used all all the electric seat jazz too. Anything can be made to work, just be sure it's secured properly to the floor.