I have a 51 which should be about the same inside space as yours. First thing I did was to cut a story stick that would represent inside seat riser space the donor seat will occupy. Then take it to the salvage yard and you are guaranteed it will fit the space. In my case I chose a 40/60 bench from 2001 Buick LeSabre with built in seat belts. I did a little fabrication work on the metal base of the seats and it fits nicely into the original seat riser space.
Hope this helps