To easily weld sheet metal you to use .023 wire w gas and a welder that that is infinitely adjustable rather than fixed steps. Otherwise burning it up is all to easy....the copper backing behind helps too.

Take care,
Rick
68 Coronet R/T 440 & 68 Charger 528 Hemi,and 5 Challengers! 6 cyl, 318, 360, 383, 451