Have a 1950 3600 with the original seat frame less the springs. Looking to rebuild to the original but haven't had any luck finding a good replacement spring option. The best I have found so far is full seats selling on ebay but didn't know if anyone has seen a part provider selling replacements or have had luck with other year springs working.
there is at least this one on ebay.. you might want to print these quickly..as in looking at it.. if you have a bare frame. you should be able to find one of the few mattress manufacturers and get them to create something from their components.. i have not checked the many after market parts sources.. these are listed for 47 to 54 chevy and gmc trucks..
you might also want to check out this article on seat foam carving for light aircraft...well written.. have a seat and read some..http://members.eaa.org/home/homebuilders/building/interior/4Carving%20Foam%20Seats.html
some studying how seats are assembled in the junk yard.. you might be if you are going to do your own.. or have a shop do the seats.. you could have them create some seat ... i forget what they call them.. indents that prevent the seat of your pants from sliding around quite so much.. before walking into a shop carrying those rusty seat springs...see if you have a sand blasting company who might do some rust and scale removal for you.. and have a thin layer of paint or primer applied.. so it does not leave RUST streaks or dirty hands when they touch it.. you will find much happier to you responses when you show it to people.. how about the 3rd row from a mini van seat... or even 3rd row seat from a full size van seat.. or the 4th row even from a full size van seat.. i don't know off the top of my head about seat widths... take a cardboard pattern or a hallway runner cut into a pattern to unroll across the junk yard seat to check the length.