Water pump drive shims are readily available. You can hone out the pulley you take off of you alternator to a slip fit to use it to mark the best position for your replacement pulley. In extreme cases you can get a mr gasket spacer tube set for the alternator, but I look for other solutions before fighting with that unless it’s a scratch built ground up drive setup. The less possible shimming and manipulation you have to do the easier it is to keep everything running true.
Summit Racing® Water Pump Pulley Shims SUM-310121