20 gauge is pretty thin, but the China metal belly is thin too, use what you got. When I grind the different pieces to make the exhaust fit the frame I draw 2 line acrossed the joints on each side to line it up for welding. Then tack weld it together, see if it fits then weld it up. I got the 2 7/8" ID diesel truck pipe ext. at the auto store for $20, can make 3 or 4 pipes from it, and it's 12 ga. or something about .090" thick. I got some 1/4" 6061 T6 aircraft alum sheet pieces got to make 3 to 4" extenders between the rear arm and frame and move the rear wheel back. Then you buy a longer rear axle kit with with billet pegs for $20 from SP4less online, and put you feet back there.
I put 10 speed toe clips on my C1.