Michael,
MIG welding thin sheet metal is not the proverbial piece-o-cake. Two variable you didn't mention were wire size and wire feed speed. For a chore like this, solid .023 would be the ticket.
Your replacement pan is probably new and clean but if the old sheet metal is rusty or funky, it will not weld nicely.
With the machine on a real low setting, and wire speed moderate, you should be able to stick two nice clean edges together IF they are overlapped and held tightly together. I don't think I would even consider a butt weld. These are going to be short tacks, not long stringers.
If this is your first attempt at MIG welding I applaud you. It's the toughest task you will encounter in the shop. Dink around, turn the knobs and you will succeed. The weld will be comprised of short beads that eventually join together, although not necessarily so.
Stuart
