Worked on my seat this weekend. I could not live with the springy original seats but I wanted the seats to look mostly original. With the help of DanTwoLakes, I came up with this solution.
I built a 3/4" plywood base for the bottom and two 1/2" plywood seat backs with brackets to make them mount in the original frames.
I ordered a sheet of 3", 4" and 5" firm foam to try. I ended up liking the 5" foam in the bottom and the 4" in the backs. This will fill out the frame and make the seats look good and sit good. These things are better than my sofa in my house and I have the seat and steering angle to my liking now.
I had to make an adapter that moved the seat back a couple inches and will make a block that will get the inclination like I want it.
The seat frame will still need to be blasted and painted. Then, when I'm ready, I will take it to the upholsterer and let him finish trimming the foam and do the upholstery. I'm pretty happy how this turned out, so far.