you don't need anything near as heavy as 14ga. for a floor, especially if you're going to install heavy frame ties!!! I drag raced a number of Mavericks and Mustangs over the years, usually used 2 X 3 11ga. for the frame ties and 1 1/2" round tube for crossmembers, with a bit of carefully placed, welded, and gusseted 10ga to use for 4" X 4" plates where the crossmembers go in. One round tube works great as a front attach point for a set of ladder bars, and another with a bolt in center section for a transmission crossmember. As your brother-in-law works in a fab shop, some nice 20ga. with a few beads rolled in it will make a great floor, be plenty strong, and way, way lighter then a slab of 14 ga. My last Mav ran low 10's, 9's on the bottle with the same setup for floor and frame ties, plus a 10 point cage! Leave the heavy stuff for tanks, takes horsepower to move a lot of extra weight!!!