Quarter Panel repairs- cutting out the bad metal
When doing a job like this, you have a couple options: you can get a full replacement quarter panel, a reproduction of what Ford used to build the car 50 years ago, or you can get a "skin".
The quarter panel forms the trunk opening as well as the door opening and goes all the way up to to the roof line where it's leaded to the roof panel. That would be a lot of metal just thrown away to fix a hole in the wheel arch. And they're pretty expensive. But the edges are well-formed and it's the most accurate replacement part. Replacing this panel is a massive job, but requires less welding.
The skins are meant to just be used as patches, not full panel replacements. As such, they are much cheaper than a full quarter panel, but the edges of the skins are not well formed and not useful for actually installing on the car. This panel would be chopped up to fit and then butt-welded around most of its perimeter. If I were a better welder, I'd be using a skin and cutting a smaller piece out of the car.
After a long time thinking about this, I decided to cut out the majority of the quarter panel's vertical skin from door to tail and as high up to the body line as possible, and use a portion of a real quarter panel as replacement metal. This way I preserve as much factory metal as I can, and get to use good edges on the front, bottom, and rear for plug welding to the car. This will leave me, however, with a five-foot-long weld at the top of the panel that will have to be done so as not to cause a lot of distortion or need gallons of body filler to hide after the fact.
For replacement metal, I could buy a full quarter panel for a fastback, but as it happens the quarter panel for the coupe is basically the same piece up to the body line, but a lot cheaper. So I got full quarter panel for a '67 coupe and use it to create a sort of skin panel that has good edges that can fitted right on the car. I will trim off the coupe-specific metal at the top of the panel so I can fit it onto the remaining panel on the fastback.