/6 Possibilities

You really cant get any Major gains with out taking the head off.
That being said go to 2 1/4 inch exhaust and super six(2bbl) it. Electronic ignition or HEI, Change your advance curve and thats about all you can do.

If the head comes off then larger valves, some porting and mill at least .080 for increased compression (I go a minimum of .100). Recam it if you will have the engine out.
My02.
Frank