From the info you gave, I think you are just about in the right place for your build; you should be getting around 300, plus a bit. If you want more from the engine, you're going to have to think more cam , even before you go to better flowing heads; but with heads, a new cam will be almost mandatory, as well as more carb, or the gains will most likely be minimal. Of course, one change begets the need for another - it's a never ending story.