Smaller water pump pulley

Need a little insight. I have a 72 duster with a 360 magnum. LA timing chain cover. I have an aluminum radiator, new hoses, 6 blade aftermarket fan, with a shroud. I haven't checked timing yet, and I don't know which water pump the car has. So I know those are both variables I need to check.

Going down the road the car holds steady at 180 degrees, as soon as I idle at a stoplight it creeps up to 220+ and then comes right down once I take off again. From reading on here it seems that your water pump pulley needs to be smaller than the crank pulley, which makes perfect sense to me. My water pump pulley is definitely the same size as the crank pulley.

I'm using a stock 29518-36 pulley. Does anyone know where I can get a smaller pulley

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If that is a flex fan get rid of it and go to a steel 7 blade fan.
You can get a little better flow by using a high flow thermostat. I use 160* Milodon or Stewart high flow T stats on my iron head 340.