I second that on buying new heads if you want more hp, but do what you want to do with the engine, and if your budget doesnt allow it, then wait until it does. You'll save money in the end and be happy with what you got rather than having problems and getting frustrated with a thrown together sbc. Spend your money once...just a suggestion from what I learned from years of reading on this board and others and past prodjects.
Check out the summit heads thread