For the seat mount, it sounds like you're (effectively) bolting the seat to the floor with some fancypants DIY seat rails. So I'd think the normal restraint bolt requirements would be enough - grade 8 and not shitty looking.
If you plan on adjusting it a lot, make sure your WHOLE crew is practiced at doing it, and maybe mark off the appropriate settings for each driver. You'll eat time LIKE CRAZY at driver changes otherwise, and it's a safety item so it totally needs to be done right every time.
For the seat support, you're in the clear with that support bar.
Driver, Pit Monkey, Rod Buster and Engine Fire Starter
Team FinalGear