If the heads are crappy 882 or 624 castings start with better castings.
#441 487 920 etc or do the low buck ported 305 HO #416 head deal.
vortec heads would scream on this motor too..
Be sure the HEI distributor is seeing a full 12vDC power.
The ignition 12v + wire on that old truck is a resistor type for points.
Be sure the engine is grounded to the truck body with a ground wire to the firewall.