A/C water pumps were driven faster than non-a/c water pumps.
35% faster in 1968
The faster they are spun, the more liquid flows and the likelyhood for cavitation increases. Those are both minimized with the smaller impeller.
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Ignore the yellow markup. That was for someone else's question.
The big issue with A/C is not the additional load on the engine, which is real, but only a significant load percentagewise at idle rpms.
The big issue with A/C is the heat exchange with the air - the same air that goes through the radiator. Its critical to keep enough volume of air moving at idle and low speeds.