just my 2cents, drop the fuel tank if your have not yet, pull the fill tube, cut the fill hole out of that qt leaving aprox 3/8" of metal all around it. rebuild the inner panel with a new piece , redo your spot welds to the trunk floor to re hook it , now cut the rest of that mess up to the upper body line about 1/4" under that line , take it {the cut} forward to the vertical line up with the rear of the wheel arch and then back to about a inch ahead of the tail lamp pocket. buy a sheet of flat body panel steel from keystone ect. barrow or use a metal break to crimp your lower body line along the full patch you cut.
under lap that piece with the top cut and hold in place with clamps , snap a chalk line to get your lower body line marked out for the metal break. trim the piece into place while clamped in.
tack it in a few spots and then cut and build the wheel arch piece.
short of finding a good used qt to patch with I think you need to remove all that damage. once you get to dolly and hammering that its going to get all oil canning on your , prob is already from prev repair.
if you work with whats there you will have a 1/4 inch or more of tiger hair & filler on that panel to ever get it straight.
I know it sucks but in the end your will have a better panel to work with and a lot less filler, take your time and don't be worried about a screw up or two , the panel sheets are pretty cheap, just don't weld it home until your sure.