Probably not much help to you,but in my Model A I'm running 3/4" marine grade plywood with the bottom sheatherd with abou 16 gauge sheet metal for extra protection. Course there's also not much floor in a Model A coupe.
18 gauge cold rolled with a few stiffening beads rolled in is all you need.... and don't try to weld things solid, you'll only end up with doors that no longer fit. Tack welds (with good penetration) every 1/2 to 3/4 of an inch will be fine... seam sealer to follow, primer and paint.....viola
I'm doing the floor in a 56 chevy g***er for a friend right now. I always have good luck with 18 gauge and a few beadrolls.
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