73 Hornet 258 Water Pump Pulley

This picture brings back memories for sure.  My 73 Gremlin had the same setup...and the same issue, where the back-side of the PS belt rubbed lightly on the water pump pulley. No, the single belt pulley you have is correct. the issue, if I remember correctly, (and this is going back a good 30 years) was that the PS pump belt was too short, and that caused the pump to be tilted up slightly, causing the rub.

IIRC, I used a slightly longer belt (like maybe 1 or 2") which allowed the PS pump to lay back like it is supposed to, and the rub went away.

The other thing to check, before you buy another belt, is to confirm that there is air-space between the belt and the WP pulley when the engine is off.  If there is, then the contact is only happening when the belt flutters (typically when at full lock left or right, or when quickly turning the wheel).  If that is the only time it is happening, then all you need to do is check belt tension (don't over tighten) and drive the car.

There's a reason AMC changed PS pump mounting in 1974, with the pump moving to the driver's side, with a much better designed brackets. This issue was pretty common.