I know my car has less than 6" of ground clearence. Probably not much more than 4" The key to me would be NOT to have the
oil pan or gas tank be the low points. In my case frame rails are protecting both. The fiberglass up front would be the first to hit anything, but after 40,000 miles it's still in one piece. I do seem to scrape the bottoms of the sidepipes about once a year.
I have no direct knowledge about mounting the brake pedals. Just be sure the floor does not flex when you push on the pedals.
Harvey R.