Biggler carburetor such as a Holley 750 or 850, free flowing dual 2-inch exhaust system, advance the ignition timing.

Of course there is a lot more that can be done with these engines, but that's about all anyone can offer without more detailed perfomance requirements from you. Porting the heads, for example, can enable them to be capable of supporting more than twice the hp that your engine currently has.

Paul