IMO TIG is the way to go with thin sheet metal, but expense and skills involved is more that a good sized MIG unit. I have a Hobart 190 220v MIG and use .025 wire for sheet metal. Then tack every inch then fill in the dots, works well. I use a cutoff wheel to smooth out the weld dots (works better then grinding wheel) before the next pass of dots.
1951 Chevy 3100 "Project for my Wife" - Jigsaw Puzzle 1969 Camaro SS350 convertible 1969 Camaro 327 convertible