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Just to help out a bit and give advice.
If you do have the welder(with a bit of experience) and you are on a tight budget, it is so much easier to take some mid grade tin and just cut the section you need out. And then measure it all out and attempt to make a good mold out of it and patch it up in there. Not saying you would use this method, but never use bondo, just because the rest of the metal will rot out and you wont notice it because of the bondo, and then youd just have to go in and redo it all again. For me, im going to be having to replace my wheel well, but best of luck to you.