Have you tried tighter lash? I would start by going 18 and 22 and see if the car likes it. If it doesn't like it reset the intakes and see if it just needs more on the exhaust side. What is the cam in at? Very heavy car might want the cam advanced. I would definately look for some better aluminum heads, and try and set the motor up for the max compression you want to buy fuel for.
Look for ways to lighten the car. A disc brake conversion will drop 35 lb I believe, plus it should stop better.