Also, put some more timing in the engine at idle.
15° advance on a performance engine is not enough. You'll need at least something around 24° imo.
Pressure in the cooling system is caused by the coolant and the present air expanding due to heat, not by the waterpump 'creating' pressure because of some restriction in the thermostat.
The pump is not a positive displacement unit. It just flings water outwards and thereby sucks water in through it center.
Pumps can cavitate if they try to peddle the water faster than it wants to move.
I've tried a number of waterpumps on a car that wanted to overheat easily. Factory stockers, 440 pumps and a Mancini pump. The one that worked best was the factory stocker with the stamped steel impeller.
