Well, I just remember that both of my 273's (factory A/C~P/S cars) had small diameter water pump pulleys. I think that's related to the fact that they WERE factory A/C~P/S optioned cars, and this is back on the cast iron water pump cars. I don't have one of these pulleys in my stash right now, but maybe if someone else does, they can post a pic with dimensions.
Also, if you think about it, that smaller diameter water pump pulley will spin faster......probably for a logical reason.