Well I just went through this water pump swap trying to cool off my 1969 Grand Prix. Car is stock with AC, 428 and stock shroud and a new heavy duty fan clutch. I did install a 28" Griffin aluminum radiator and a 180° Mr Gasket thermostat.
Anyway, it always ran hot especially with the AC on. I just replaced the pump with a new cast impeller GMB pump and I hammered the divider plate to get about 1/"16" clearance. The old pump also had a cast impeller and had about 1/4" clearance to the plate.
Today driving at 75 mph with 93° ambient temperature, the engine coolant temp is 198° with AC off and about 210° with AC on. These numbers are unchanged compared to the old pump.
The motor is fresh so there is no issue with blocked coolant passages. I am running 70/30 water/coolant mix.
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