LONGBOX,
I reckon anything can be fixed...but at what cost in time and money? If it was made it should be repairable, if one has all the metal stretchers and rollers, etc. Real world is that I hate to think of spending almost as much of a pair of fenders as I did for the whole truck!
Here's what I'm envisioning: A steel patch panel arc that is the same shape as the fender well opening that could be spot welded into the inner surface and probably wire-fed along the outer edge. Then ground smooth and covered with bondo to get a real metal edge. I'm thinking that someone with the right equipment could make such a piece...but it appears not yet. Maybe I'm the only guy with the arc rusted off my fender?

I used the cab patch panel corners back in 1966 when I fixed up my first '50 1/2 t. I was hoping that patch panels had progressed far enough to bale me outta this one!