I have a few seat foams I robbed out of school bus seat from work. They are quite dense and about 4" thick. The foam is not a solid one piece, but lots of small crumbles molded into one slab, I believe it what makes it stiffer. Back cushion foams are single injected types of foam, softer and more pliable, and thinner.
You will want a stiffer foam for the base and outer edges with a softer foam in the middle, so when you set it feels similar to a bucket seat. A one piece slab won't have much feel or control to it. The stiffer edges will help keep its shape longer. The bus drivers seat foam is an injected foam, after a few years, they are just flat foam, with no shape left. I would also want an open bottom to the plywood base with heavy webbing for a little give and air flow, similar to a lawn chair.
Go to your local foam dealer and stack up a few different grades and set on them, keep swapping grades till you find a combination you like. We just bought a new bed mattress, one of the latex types with three different firmness to choose from. I like a firm mattress, so one side has three layers of firm, the other side is one firm, one medium, one soft. It feels really good and you can really tell the difference in how you stack the slabs, we tried a few variations. Your seat base needs to be the same way, you just have to try a few to see how you are going to like them. Theres nothing worse then an uncomfortable seat on a long drive.
My '37 seat frame was decent but the outer edge springs were worn out. I had no support under my knees and leaned towards the door. I cut some of the bus seat foam in to small slabs and shoved up through and around the seat springs to stiffen the edges. It made a huge difference in how I felt on long drives. No more back aches or stiff knees, and when cornering, I don't feel like I'm going to fall out anymore.
