I know little about seat foam, but when restoring my '37 the seat springs were too deteriorated to use. I only had the seat base frames. This truck has a split front seat. My upholstery guy suggested using high density foam, which ended up being 5 or 6 inches thick. Seven years later the seats look and feel great. So the seat construction is vinyl, foam and a plywood base attached to the seat frame.
Kent


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix