351 Cleveland power steering

On mine the belt drives the water pump and power steering.
the alt belt is alone in the rear groove on the crank.
But I have A/c.

A Guess

I'm thinking you have the wrong pulley for the water pump. I does not make much sense to have different sized groves on the pump than the crank.

I just looked at my 72 351c 2v. The back belt, closest to the motor uses the smaller diameter groove in the crank then water pump then power steering pump. My water pump pulley only has one groove. The next belt is crank (larger diameter) to Alt only, nothing else. I have a third groove (same diameter as Alt) on the crank for the A/C but not in use at this time.

I'm sure different years with different options have different layouts so this may not be any help at all. Maybe find a different water pump pulley or order a set of under drive pulleys, that way you have a matched set you know will work.