Patching with a MIG is always good advise however be prepared for a small problem becoming a bigger problem. Will take a pro's touch to keep that thin rusty steel from becoming swiss cheese. I always like to fix holes with steel but when I risk opening up a can of worms I resort to canned fiberglass. I too am fixing similar holes in my 55-3/4 ton this weekend. Found some fiberglass paste made from milled fiberglass rather than short strand. Should be easy to work with. Fiberglass over epoxy primer. Good waterproof fix.
Good luck with the repair...and remember to have fun!
Keith
