Big block rebuild - where do you buy your parts?

Now this all depends on how your chambers measure now. I agree with @PROSTOCKTOM that milling heads for compression isn't the best idea. If you have to go from say 90CCs to 80 for example, that's probably a .060" cut and I don't recommend it. .030 would be about all I'd go. That and using the thin head gasket will be fine.