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