Was catching up on threads... found this interesting, If I understand the layout correctly, I think these assumptions take up too much space.
I've found that if the rails are relatively close to the tubes running the length of the car and you have a center piece of tubing you simply bond and rivet .050 -.060 aluminum as the floor and just weld 1/8 " thick tabs with the seat rail holes in them to the 1.5" square tubes, flush with the bottom, sitting on top of the aluminum. Some prefer a sheet of more resilent steel under your butt for safety. I think that is how the GTM does it, a sheet of steel...
This is how many Seven clones are made. In my reverse trike I did this, the tabs are 1 1/2 inches sq. 1/8 " steel, and have seat rails that are only about 3/4 of an inch thick. Purchased my seats and rails from Cerullo http://www.cerullo.com/sportsSeats.html in CA. The entire assembly leaves the bottom of the seat about 1 inch above the lowest portion of the chassis, the flat floor panel. Use a very thin bottom on the seat (perhaps that Aeron stuff from office chairs) and you could really maximum headroom for a "wookie". At 5'8" that's not one of my problems, I can't see very well out of the F5 GTM Demo!