x2 on Pete's videos in general. Very informative and if you have ever talked to him, one of the nicest and knowledgeable.
My car was rotted like the 81 you are looking at. After getting under the car and removing the rear end, all lines, suspension, fuel tank, rear bumper etc I found a lot of rot. I ended up replacing the rear quarter extensions (behind the rear wheel), the trunk drop offs(these are the overhangs on each side of the inside of the trunk and forms a pocket with the quarter extensions that trapped moisture dirt shot gun shells, whatever LOL!), rear portions of each frame rail, inner tail panel and tail panel. I have posted many pictures of the build- Check out my resto page here, it outlines it pretty well.
http://www.78ta.com/HTAF/index.php?topic=34786.0 It was a lot of work, but is something one can accomplish in a garage with pneumatic and electric tools, a good welder, jack stands and ramps.
The cowel, the toe pans, radiator support, trunk pan, under the molding around the windshield and rear window are all other places to look for rot. Hope this helps