As RUSTY Cuda explained in that other thread, the foams on those seats are formed to the steel frames. On the 70/71 style, it is a separate piece of foam that slips onto the frame. Before I found my 1970 seats, I had some 72-74 style seats that came with my car - I have attached a picture below, showing what they look like with most of the foam peeled away. (You can see my old seats in the For Sale forum).
The best suggestion I got for those later seats is to take them to an auto upholstery shop. They apparently can peel away part of your old foam, and mold/shape newer foam onto them. (They can sort of 'build up' the foam to fill your seat covers). I have no idea about the cost.
Tom