I'd go with a large 385 series BBF. It's very easy to get 700-800 N/A horsepower from them.
Check out this one on Keith Craft's site:
http://www.keithcraft.com/crate_engi...00_competition
The last time he had dyno numbers, it was north of 800hp naturally aspirated.
Jon Kaase makes big power with his heads and 385 series engines. There's lots of potential there...