Joe, not a NAPA number, but here is the water pump you are looking for at RockAuto. I bought one recently...new, not rebuilt.
http://www.rockauto.com/en/catalog/chevrolet,1957,truck,3.8l+235cid+l6,1326091,cooling+system,water+pump,2208
The easy part is finding the water pump, the hard part is finding the thermostat for the bypass housing. If you do a search on this forum, you'll see threads about the thermostat issue.
Matt
