And why wont a spacer work on a clutch fan? It simply goes between the pulley and the mounting plate of the clutch, instead of going on the fan. I've used them with clutch fans before and the base doesn't know if it's sitting on a pulley, or a pulley with a spacer.
Just don't go too thick on the spacer when using it with a clutch fan. I'd not go more than 1.5"