You've been given some good advice by these guys. One thing that really puzzles me is your statement:
"and later get a bigger car that would deserve a big V8..."
The whole idea of going quick and fast is to have a lightweight car, and an engine that's capable of propelling that car to your expectations of speed or elapsed times. Get yourself a nice 360 and rebuild it to the level of power you want or can afford, THEN, and only then, replace the /6 with the 360. Voila....instant muscle car. :)