1966-67 Tempest/LeMans
If the right one was close enough, I wouldn't be driving my Firebird. Nothing is more BA the a 1967 Burgundy GTO pontiac A-Body. Parts aren't too easy to find. Get an interchange book.
1968-70 Tempest/LeMans
Parts are slightly harder to get.
1972-74 Challenger
If you're one of the few people that can stand the challenger's later looks, good for you; however they don't seem to be so cheap anymore.
1967-69 Firebird
I have one, love them. It's the Camaro's classier, more affordable cousin. Parts are pretty easy to get, second to the mustangs.
1970-1973 Firebird
Not that much cheaper than the 1st gens, parts are harder to get. But if it does it for you.
1974-1978 Firebird If you avoid the trans am, these cars are cheap. Get an esprite.
1964-1968 Mustang (not-fastback)
IMHO nothing is more American, or more diffecult to screw up. So many sources for parts it would be the easiest car for you, but a 289/302 or even 351 is by no means a pavement pounder.
1970-71 Torino/Rachero
Ford's Chevelle and Elky, a whole lot cheaper, but parts are
diffecult to find. (Diffecult=expensive)
1967 El Camino
1968-69 El Camino
1970-72 El Camino
Elky's are just cool, although you'll never stop hearing Portoriccan jokes, also... no back seat
but 68's on actually heavy better weight distribution than chevelles.
PS. You're in my situation: soon ur friends who wonder why don't you just ask mommy and daddy for a bimmer ('cause I'm not loaded), and tell you you're buying old junk start to tell how jealous they are.