I do not believe any one has mentioned if the car has original quarters on it?? You may want to see if it has had quarter skins put on it. Quite easy to see a welded seem inside from the trunk. Those repro skins have extra metal that is meant to be trimmed off. A lot of guys either tuck it behind the tail panel, or just weld it right to the tail panel and then bondo over the hole thing. I find when the body line between the quarter and tail panel is smoothed out( like yours), that is usually the case. You may get lucky and the tail panel is alright. I would say pull the bondo off where it is already breaking loose and see what is going on. Atleast then you will know if a patch panel will work out for you. Worse case you would have to fiberglass it up and smooth out with filler to get you by until you can fix properly. Hope this helps,Tyson