[chevyrestoguy]

The comments about using gas is definitely correct. The cleaner the weld, the better. Looking at your picture, your gaps look OK. Because you're going to be grinding the welds flush, a small gap is desirable because you're going to be grinding off 90% of the weld getting it smooth. Butted panels with no gaps works great when you're using a TIG, but with a MIG, a slight gap is best. You can buy panel clamps from Harbor Freight that work really good, and they're cheap. If you can't find some, let me know, and I'll loan you mine.

Blue Healer has a great tip on filling gaps. Brass, copper, or aluminum all work great. You can use scrap, and clamp it to the end of a pair of vice grips, or in a pinch when the access sucks, duct tape it into position. The heat will make a gooey mess of the tape, but the function of the scrap will have done it's job.