Something doesn't look right
I had Legendary upholstery professionally installed on the buckets in the 66 GT I restored. The bottoms were massive blocks of foam over a steel frame, no zig-zag springs. Similarly the seat backs were all foam over a rigid frame.
Neat trick I learned for split seat backs. Get the springs and foam all in shape for covering. Cover with a leaf size plastic bag. The upholstery just slides on over the leaf bag. Otherwise it's like stuffing a size 8 girl in a size 4 swim suit. When you have the upholstery in place just rip the leaf bag out from under it.
Best of luck. Keep us posted.