Jim,

It looks like the 235 and 228 water pumps are different.

Here is a rebuild service.

NAPA also has the rebuilding service for 1941 >> GMC Truck >> 100 >> 3.7 L 228 CID L6. Their on-line catalog is not easy to link to. You'll have to look it up:
http://www.napaonline.com/MasterPages/NOLMaster.aspx?PageId=430&CatId=5&SubCatId=4

Tim