Tim,
I don't have to answer to stock pulley sizes,but....
What diameter is your water pump pulley ,6" or 5.25" ?
6" pulley is 4% underdriven
5.25" pulley is 10% overdriven
March will sell you a water pulley by it's self.I did that.
If you have the 6", 14% increase in pump speed could help.
http://www.marchperf.com/pg37.html