One thing to consider is that if you rebuild the seat properly, with a high quality batting kit, it can be more comfortable you might think. I was about to replace all my springs with plywood and foam, but at the last minute I decided to rebuild the original seat with the best kit I could find. And other than not being happy with the vinyl cover kit that I bought from LMC, the seat is pretty nice to sit on.
I got my seat padding kit from these guys: LeBaron Bonney
Catalog page: Seat Padding Kit - (large size)
They've also got a few good instructional videos on seat rebuilding.