there was at least one through-the-floor anti-sub install which employed a massive backer plate, through which, even the lap belts were mounted. it spanned the entire area between the frame rails. i can't remember who it was, but it certainly was impressive.Bolting the sub straps through the floor pan under the seat is a REALLY bad idea. The floor pan is just thin sheet aluminum and will tear out. Even if you put a reinforcing piece below it, there is a good chance the floor will still buckle under the loads during an accident. It would be better to either go 4-point ASM or get a harness/seat setup that allows you to run the sub belts back to the lap belt mounting locations like the Lotus Sport setup. If you are running stock seats, you can look at the sub strap bar from S111, but I consider that as somewhere between a proper set up and the floor pan idea.