You can run a 5/8" pulley on a water pump on a 235 or 261. The older 235's had 5/8" pulley and I have run 5/8" pulley on a 261. I even ran a water pump with a double 5/8" pulley. It looked funky, but it worked.
Your question can best be answered depending on what vehicle you have the engine in. You can go either way and I would go the cheaper way, but you may not be able to go that way if your vehicle restricts you from going that way.
So - what vehicle are we talking about? :confused:
