[jbarron]

Add more initial timing.

I had the same issue. I'm more like 18-20 initial 36 total(mechanical), vacuum advance adjusted to just keep it from pinging.
Carb could be a little lean, check the plugs. They are not perfect out of the box.

Went back to factory AC water pump. The HV pump was doing more harm than good.
A 165 thermostat will give you more temp room to play with. Also only use one vent hole 1/8" if there is not a check valve in it already. The water flow around the tstat affects its opening.

Mine is one of the GM 290HP crate motors with eldebrock 600 carb. Added vortec heads, but heating problems started with originals.