You can use 16 gauge if you wish, but what are you going to do when you reach a point where there was a reinforcing piece inside? The 16 gauge isn't as strong as 2 layers of 18 gauge. Also, the original factory frame relied on the frame flanges and a good floor pan to reach its strength level, how is your 50+ year old floor pan doing, is it just a little weak? 16 gauge is pretty tough to work with (14 gauge is worse), its too thick to cut with hand held tin snips, and its pretty difficult to hand form. You have to buy 16 gauge in sheets, so everything has to be cut to size. 11 gauge can be bought in specific widths, as well as in box and round forms. 11 gauge is actually easier to work with. Often 2" x 3" 11 gauge tube slips inside the original frame rails. Its a lot easier to get a good weld on 11 gauge then it is to get a good weld on 16 gauge for most non-pro welders.

I can drill a hole in the bottom, and cut an access hole in the side of a 2" x 3" 11 gauge tube to install a nut to bolt up a K member and not be concerned about causing a weak point. You can't do that with 18 gauge, or 16 gauge, or even with 14 gauge. Build your frame repair pieces with whatever you choose, I'll do the ones I do with 11 gauge. Gene