I second GoMangoCuda's opinion. I'm working on a car that didn't look half as bad as your pics and found out the rust was twice as bad as I had thought. You'll have to cut back pretty far to get to steel that you can weld to. Because that is a structural member you're probably looking at something like 18g steel with some tight bends. I've been working 18g steel to fix my rocker panels and its noticeably harder to work with than the 20g I've been using for body panels. This is my first attempt to do panel forming and it takes a lot of practice so be prepared to have to make the piece a couple of times before you get it right.