Cooling problems

Would also help to know what diameter your crank and wp pulleys are. If wp is spinning too fast, the coolant may not spend enough time in the rad; too slow and you're not pushing enough coolant through the engine... it's a fine line.
I also don't like the statement "gutted thermostat"- the symptoms you describe (if I'm understanding you correctly) are typical of not having ANY coolant restriction (t-stat, restrictor discs). You need to get a good quality HIGH FLOW thermostat in there.
Pics please.
BB A bodies are notoriously finicky about cooling- and you added 68 cubic inches and a supercharger (=even MORE heat) into the mix.