The stock pulleys are on it are for the NON A/C setup. They are fine if everything else is working properly. They were designed to work at that speed and coolant flow rate without overheating the engine from the factory. If the car is overheating now 50 some odd years later then it ain't the water pump pulley ratio. It's something else. Jus sayin. If you want more flow, then just install the 8 bladed style water pump on it if it doesnt already have one.
Your problems are multiple, but it ain't pulley sizing or trying to speed up the water pump. Especially on what appears to be a mild street 340. Contrary to what others are saying, overspeeding the water pump for flow is a bandaid on the problem if the radiator is partially plugged, not a fix. From pix and inlet/outlet it appears to be a V8 22" radiator. Verify the engine temperature with a digital point n shoot thermometer on the radiator, and on the engine at the thermostat. Once you know if it's running cool and the gage and or sending unit are off, or if its overheating, then take it from there.
A electric fan and mechanical fan fighting one another for airflow
B a possible partially plugged/ or undersized radiator. Get it unplugged, or recored, back flush out the engine, and heater core while at it
C ditch that flexi fan for a good clutch fan with an 18" diameter fan blade setup.
D put a good new 180° thermostat in it
E put a good radiator cap on it.
I'd place my bets on that radiator possibly being partially plugged, and there being possibly a too high of a temp rated thermostat in it.
Good luck.
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