If any forming needs to done then 3003H14 for flat stuff with minimal forming the its 5052H32
3003 forms well and you can aneal it back soft after it work hardens whereas 5052 does not aneal so simple forming with a brake or roll is all you can do, its really stiff and 30% harder to work then 3003, but you can get away with using .040
Depending on welding skills .050 is 50% harder to weld than .063