I have the stock fibreglass shells in my Striker, which i got recently. I took the drivers one out last night, it's bolted at the front edge to the floor and the top of the back rest is bolted through a piece of 2" x 1" to the rear panel. The seat appears to be 'suspended' above the floor at the back. Ie it's only really touching the car at the front edge and the top of the back rest, is this right? I deally I was thinking of putting a larger spacer behind the back rest to effectively push me forward a little. I was expecting to have the pack the bottom of the seat to the floor, however I didn't expect it already to have a gap under it. Should I fill the void with expanding foam, I was thinking about putting expanding foam in a plastic bag, so it provides a cushion, but doens't stick the seat to the floor.
How does everyone else do it?
My other thought, more long term, was have some leather drop in liners made that have more padding on the upper part of the back rest, I don't like the laid back driving position really

Its quite normal for GRP seats to be only mounted at the front edge and somewhere up back of the seat. There are options for mouting them at the base only. Check the JK website for their options:
http://www.jkcomposites.com/7s-page.html

I have JK composites seats and these are mounted at the front edge, but I put twin-pack foam under and behind (in bags).

I did a rubbish job so it looks a bit crap, but it works well and gives good support.