When I built my 250 I replaced the water pump.When I was putting the belt on after painting everything and installing motor,installing radiator,grill,and at last the belt..oh no..a miss match.My first thought was that I had put the SPACER on the alt. on the wrong side,but it turned out that there are two different sizes of water pumps for 250 chevys.I took my old one to the parts store and did a cross on the casting number and came up with the right one.If you used the same balancer it has to be the Alt. spacer or the waterpump SCRAP
