'65-66 Shelby Mustangs are easy to make - a few scoops, some stripes, sparse interior, and a couple bolt-on engine parts are about all it takes to "look" like a Shelby. They're all over the place actually.
'67-70s are a lot harder to fake because of some replacement body parts (fender extensions, bigger scoops, etc.).
I've worked on a lot of early Mustangs and they are dirt simple to work on. Unless you get one that's been repaired - you'll always find rusted floors and door bottoms (easy to fix) ... everything else holds up well if you find a southern CA car. the inner cowls can be bad if the car sat under trees for a long time (junk collects and traps water). Camaros are about the same, but you need to worry about the rear window channel and package tray ('60s GMs are notorious for rear window channel rot).
Any Mustang questions ... just ask ... I've totally restored mine and have worked on just about all the others.
Gratuitous pic ...
