The WJ is a good pump, no reason to replace it for a moderate performance build. 

I'm running the original Jacuzzi "B" impeller in my WJ in a similar sized boat behind with good results.  The main thing I'd recommend is not to spin the Olds past the 5000rpm range for more than a few seconds at a time.  Extended high rpm seems to be killer on that engine without major oiling modifications, and even then it's limited.