Roger,

What you described is pretty much the method I used. While I expect the body to stay attached to the frame in case of an "incident," I did not want to be the "cotter pin" that held 1000 lbs of body to 1800 lbs of engine and frame via the seat belt. If God forbid, the body comes off the frame, the seats will still be attached to the body, as will I, because of the seat belt.

I used 4 plates per seat. Each plate had a hole drilled in it with a grade 8 bolt inserted. The head of the bolt was tacked to the plate. Then, I inserted the bolt through holes in the floor pan and glassed them in place. That way, I had studs sticking through the floor, and didn't have to crawl underneath the car again to install or remove the seats.