OP, you're probably going to find damage to the inner fender too. You should also know, if you don't already, that straight sharp corners are not a good idea, as you will get sink in the panel at that junction. I learned this the hard way when I welded in new fender wells on my 86 F150. I can talk to you about oil canning too....not fun!
I am in the process of a similar repair on my 69 Mach 1 that has damage on both sides of that lower rear qtr area, however not near as extensive as yours. In fact, it wasn't even noted until I had the car media blasted, at least not on the passenger side. Anyway, I repaired the driver's side with a crappy patch panel that I bought some time back from CJ Pony and had to rework the underside of it in order to replicate the OEM piece. DonP has pointed this out in his post above. Some are okay with the incorrect underside contours, I am not. On that side, I also had to rebuild part of the inner fender and mate that mess together. That was not fun. I'm not totally happy with it and am seriously thinking about cutting it all out and re-doing it.
Since that time, I have bought another patch panel from NPD for the passenger side, made in Canada (Spectra), and it really comes close to the OEM curves. At least this one does. Still, I have much reservation in cutting out much of that panel. I've cut out a small area and even have a patch made for it but it would just be a whole lot easier to remove more, fix the inner fender area, clean up all the rust and weld in a portion of the new patch. The more I think about it, the more I think that that is what I'll be doing. Good luck in your project and keep us posted on your fix.