For a sleeper, I'd suggest a 1985 GT.
The later Fox body can really instill a sense of fear in other cars. It's not really a sleeper. Hehe, most of the Brand X guys practically think it should be illegal to mod them. It can look stock and run 10's
The '85 looks sweet in my opinion, but it's still a sleeper, has a nice interior, it's light, has a roller HO motor and a Holley 4bbl that makes 210hp.
It's low points, weak T-5 rated at only 260 lb/ft, and the 7.5" rear end, oh and they usually have those 10 hole wheels, ack.
They are pretty easy to mod, and for not much money you can get them pretty damn quick, a lot quicker than what people expect.