You should be able to get 600-700 reliable HP on pump gas with a big crate engine. Check out the 502's and 540's that are available. Some are available with decent warranties as well. I know that some people consider crate motors "cheating", and that you can do better if you build it yourself, but the crate engines really are a good value for the dollar. Sallee Chevy and Merlin are just a couple of companies that can build a 600-700 big block for pump gas. Expect to spend between 7 and 9 thousand for a complete engine.
SteveM.