Mig is wire feed. Your best bet is to have one that has a hook-up for gas (argon) This will make for a lot cleaner weld. Also to not weld in a continous line. Jump back and forth one side to the other welding a 1/4"- 1/2" at a time. Let cool in between. It will be somewhat time consuming but will minimize warpage. Better to spend time letting it cool than to spend time trying to straighten a warped panel. Personally I would weld until it is solid but others do as 41willys does. It's all preference.