I want to reuse the front bench seat in my 36 terraplane, but the springs are rotten and garbage. The frame is good so question is can i make a plywood base for the frame cut 2 large holes in it and cover with mesh straps and then use foam then cover with the material? Is the foam going to work and if so is there a limmit to the thickness of the foam I can use?
Yes you can. As for the thickness, it will come down to headroom, and how much ummm ass padding you have haha. Here is my seat build, same thing you want to do: http://www.jalopyjournal.com/forum/group.php?do=discuss&group=&discussionid=21768
My wife did exactly that on the rear seat for an old Pontiac a few years ago. I built an "adapter" frame for her so she could mount the plywood to the stock base. Looked and worked great.
Foam comes in different densities and thicknessess...really soft all the way to "it'll hardly dent when I push on it." Check with your upholstery supply house...tell 'em what you are going to use it for...they have it all. R-
If you want original you can buy coil springs from upholstery supply places. They are still used in high end upholstered furniture.
Here`s a seat out of a 40 Ford. Got rid of the springs. I used foam out of a pickup from a junkyard. Affordable and it will feel like a seat from the factory. I just built the seat base up and the seat back to the original configurations so it would appear stock.