Really? The FSM shows OD pulleys for performance. It’s about FLOW. Air FLOW and coolant FLOW. You have to have both, and the OP is missing one or both of them.
Unless you are running down Warren Johnson for the money, the tiny bit of HP you save underdriving the pump is meaningless.
I’m sitting 15 feet from an all stock 1973 duster, factory air, factory cruise, two owner car.
It HAS an overdriven water pump.
It’s not complicated. The OP is down on flow somewhere. Speeding up the coolant is never bad.
Edit: I can take a picture of the FSM showing OF ratios. Your assumption that everything on the OP’s car is just that. He needs to figure out if it’s air flow, coolant flow or both.
BTW, the OP says his car is a 68 and the water pump is for 70 and later. So we have no idea of what else has been changed.
