38 Racing---You should be allright with the 16 gauge.----if you are using plywood over top of it to distribute the load over a large area. If you cut out an area for the transmission, I would recomend forming some kind of tranny cover from more 16 gauge and welding it in solid---that will add tremendous strength to your floor, far more strength than you will get from plain 16 ga. with a hole cut out of it and left that way.