I am working with .032" 6061 T6 for my '73. The original stuff looks considerably thinner--probably .025" at best. As for seams in the underbelly, I would suggest that it doesn't make much difference. Because my trailer has a large roughly 4' x 5' sliding panel in the middle to allow access to the fresh water tank, the belly pan is broken up into at least 6 different sections--they just rivet together on the cross members, outriggers, and frame rails. In fact, I am strategically creating a few extra sections so that if I need to get at the plumbing in the back, I can just drop a 2'x 5' panel rather than cutting into it or dropping the whole pan. Its not supposed to be water tight anyway.