Are you mounting fixed in position or using a slider?
Is it an early or late model car?
- For my driver seat with bottom mounted Sparco slider, I used tubing to make a frame sorta like you described, attached to the sill and tunnel. It's a little higher than I'd like, but this is still a shared training car so we need adjustability.
- For my passenger seat, it's fixed it in place by two 1.5 inch channel iron from tunnel to sill, then I used angled aluminum secured through them and the floor, then side mounted the seat to the aluminum so it sits all the way on the floor.
On both of them I left the front raised floor in place but cut out the rear factory mounts. That gave me the seat angle I liked. Mine's an early car.
And, WATCH OUT for the fuel/brake lines when drilling.