Anything more specific than that? Your typical muscle cars are gonna be tough to find cheap. Mustangs, Camaros, Chevelles, Novas, Challengers, Cudas are going to command a lot of money. If you want a 1964-1974 classic muscle youre going to have to think outside the box. Dodge Darts, Demons, Plymouth Dusters, Scamps, Chevy Vega, Ford Maverick, Gran Furys wagons or 4-doors instead of 2-doors. Not your everyday muscle magazine stuff...
A niche that I really like that often goes unnoticed are the Chrysler 2.2 turbo cars from the late 80's/early 90's. The Dodge Rampage, Charger, Omni GLH, Daytona, TC3, CSX-T...Then later into the Spirit R/T...Shelby did a lot of work with Chrysler, those cars are remarkably fast.