I recovered seats in a Datsun 240Z I had a few years ago. I used an aftermarket seat kit, I also bought new foam. Visually they turned out pretty nice but they were too soft. I don't know if it was the foam or something I did but I wish they had turned out a lot firmer. It's not that hard of a job, IMO.
I'm going to redo the seats in my 944 when the time comes and I'm going to figure out how to make them firm like a sports car seat.
Bruce