File under FWIW, or "Yes, Gonzo is being an A-hole today"...
Every production (read "unibody") car I'm familiar with attaches the seat(s) thru the floor. Some mounting points seem more "solid" than others, but still, they're mounted to the sheet metal of the floor. In my MGB race car (also unibody), I mounted the seat to the floor, but put 4X4 inch steel reinforcement plates underneath at each mounting point. The back of the seat was bolted to the roll cage.
That seems to work for production cars, however, they have big side rails in their unibodies, side impact protection built into their steel doors, a steel roof and A/B/C pillars, and generally more "structure" around the seats than a typical 7 does.
On the Slotus, since we had those nice, strong frame rails right there any-hoo, we welded steel tabs onto the frame and bolted the seat to them. The seatback is bolted to the cage. (All grade 8 bolts, too!) Partly, the frame mounting was for convenience, but it was mostly for the safety aspect of it all. "I ain't no engineer" but it seemed like the best way to attach the seat, and by extension, attach my precious l'il white arse to a solid object.
I mostly agree with Andrew's point, it does depend on the car's intended use and the material/strength of the floor. If you're going racing (not autocross, racing) or going to be on interstates or main highways, I'd be concerned about the strength of seat mounts to the floor. My cynical side says, "Iffen ya get hit by an F-250 that's doin' 80 straight at ya, it ain't gonna matter." The good "Me" says to do whatever you can to be safe, and attaching seats solidly to the frame is a very, very good idea.
So after all that, what I'm saying is "Maybe you oughta re-consider..."
My .02, but for you, no charge!
JD Kemp
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JD, father of
Quinn, Son of a...
Build LogQuinn the Slotus:Ford 302 Powered, Mallock-Inspired, Tube Frame, Hillclimb Special "Gonzo and friends: Last night must have been quite a night. Camelot moments, mechanical marvels, Rustoleum launches, flying squirrels, fru-fru tea cuppers, V8 envy, Ensure catch cans -- and it wasn't even a full moon." -- SeattleTom