The only way to gain more setback is to use thinner material/padding on the custom made seat back. Maybe an inch or two at best.
Martin is correct. I gained about two inches by using foam padding instead of the original coil springs. The rear seat of an early 90's Cadillac donated the foam which was installed on the original TF seat frame. Looking like stock seats was not my goal. Comfortable seat was my goal and the Caddy cushion foam is much more comfortable than the original coil springs.
Mark
