I just got a two pulley one and I have to turn off .125" off the engine side I think, it has been a while since I measured it. Someone said on the 3 pulley one you need a small spacer, which they made from a piece off their old one.
Make sure you make it square so the balancer doesn't wobble. You can get rebuilt water pumps w/ the correct .375" pulley from Carquest and others. Ask for one for a early '60's 270 PU.
