Holy man, I've dealt with rust like that before, it's a challenge, but you'll feel so accomplished once you're finished. Once you're done the welding, I can't recommend POR-15 enough, buy it, use it, and follow the directions explicitly, the repair will be tough as nails. You'll also want to seam seal everything.
Keep us informed this is a noble undertaking worthy of attention!
Performance one step at a time.