Your next best bet would probably be to get a pair of GM Vortech heads; but you may want to check with a machinist to determine how much to shave them to give the 305 a decent compression ratio. They're only about 500 something bucks and make terrific power.
Your stock cam is good for torque production if you want to keep the motor torquey. A slight boost in compression and better heads, intake and carb would make a very big difference. Also consider the best exhaust you can get to fit in your truck.
