You need to get a die grinder and a few carbide burr bits and get silly on the cylinder heads to get some port air flow going on. Yu need cylinder head flow to make horsepower. Generous full home brewed porting fixes that up.
What is the casting number on your heads? (under valve cover). some 305 heads are better than others.
The ones with the tiny 1.72" intake valves kinda suck.