I did manage to get about an hour in tonight and got the wheel well patch tack welded in place. I think it will work. Now to get a few more tacks on it then get the end cap back together and on the truck.
Mike Moore was kind enough to send a few NOS wheel arch filler pieces home with me from my visit a few weeks ago. I'll use them to fashion a new one rather than using the fiberglass one that I have. That will come after the end cap is on and the inner quarter welded in.