The rear quarter panels on a Fox Thunderbird are monstrous things. How hard would it be to replace one? Are there any welds anywhere in the mix? I figure some of the anchors are inside under the upholstry. And how compatible are quarters from later models than 83? I know the body stayed pretty much the same except for changes to the tail-lights but I've never looked closely enough to judge. Reason I'm going after one is because this panel is really rusted through under the trim as well as some jackass backed his truck into it a while back while it was "safely" parked at home.
The quarter is welded to the roof panel, the rocker, and the unit body of the car. You would have to cut it off and weld a new one on. Probably more of a body shop thing than something you can do at home.