Just a comment or two.

It looks like the previous repairs included pulling the Qtr panel already. Whomever did it either had a light touch on the frame machine or used the floor-mounted puller and took it in increments while hammering. Glad they didn't pull any holes in the rear body panel, because that actually doesn't look too bad. The rear floor pan may be kinked, but back then, they weren't that precise, and it looks like the impact was at an angle, nearly over the bumper instead of bumper-bumper. Odd, because most R/E impacts like this involve submarining, and the damge is lower. I'm wondering if this was a backing accident.

Having full-size spares back then tended to save some buckling in the floor pan. If a Qtr is available, I'd run the the whole thing, even the dogleg, because that may put your seams out of sight in the door frame. That way, the sail/lower lines stay really nice.

You've got to R/R the back glass, and I'm surprised the shop didn't do that when they pulled the Qtr. You may want to let the shop know that a glass firm may be best to do that, in case it breaks. Someone knows their metal, or it could be like it looks, a backing accident. If so, I doubt the frame rail is that bad, but you'll know when you put it on the machine. The Qtr apron, you said is fine, so I'd look where the differential hump is, and if the seams aren't moved, you could be in pretty good shape with a Qtr and the other parts. You've got to pull the rear seat to get the Qtr replaced, anyway, so that's a great time to really inspect it.

I've got a used Mustang parts dealer near here, out in the country, not even sure he advertises, so I'll see about that luggage lid.

IMO, I'd skip the louvers, even in Florida sun. They tend to pool water and rust out the window frame, and there's not a lot of pitch on the rear to let the water run off. I second the vote on Cragars, because they were "the wheel" back then, especially on Mustangs. I have no idea what tire sizes would be on it now, but back, you were looking at 60's on the rear and 70's on the front, for most folks, but I always found that 70's or their equivalent all around looked pretty good, too, RWL's of course. Red lines were about out by '71 were they not?

Now, I think I have some green paisley carpet for that back window, and maybe an underdash 8-track in my attic. I've not seen any Jensen Mindblowers lately, but that would be pretty cool.... I still have a copy of Bullitt and can rent Vanishing Point....