S281, modded with a better blower, results in 700 hp. Get that one. Cheap.
Let's say you do this:
Car: $10,000
Engine: $10,000
Paint and body: $5,000
Drivetrain: $5,000
That's for an average mustang or LS1, $30,000.. That's about right for 500 horse hot rod. Thus, my advice, buy...