If you pm me with an email address I'll send you a series of pictures on how mine were done. The wheel wells and drops in the trunk also needed patching. I cut patch panels from quarter skins and fabbed the wheel well and dop patches. A member here now sells the drops if you need them. The attachment is what the lip looked like after the rust was cut out of the quarter. Your wheel well might be rotten too.
The mustang fenders have that flare from the factory on them...Those are not fabbed/added on... If anything those have been stretched a bit to accommodate the rubber/rim combo.
That is why I have a donor car. All the curves can be extracted from the donor. For shaveing doors and the like. Id think about it or fabricate with a good hammer and a piece of railroad track.
LOL, I used to have a pc. of railroad track welded to a stand back when I did body work... It was the one tool in the shop that every one made fun of me when I built it, but every one always wanted to use it...
I tried to weld a hammer and a piece of railroad track to my car once but my welder wasn't strong enough.They kept falling off.