Mfg. defect? 4.9L water pump

Hi. My cousin has a 94 Ford F150 inline 6 4.9 liter. The original water pump has a 3" impeller. The only replacement pumps he can find have a 3-7/8" impeller. The larger impeller will not work on his truck; it rubs on the inside of the motor when bolts only finger tight. Even the dealer seems unable to help, suggesting he machine a pump down. He did so... pump lasted one week. Ford (or somebody) made the original pump, why can't he find an exact replacement? As far as he can tell the motor is original. Nobody seems to have ever heard of this problem before. It is as if the block dimensions are insufficient for the pump Ford currently manufactures. Ant help provided is much appreciated.

Rob

That should take a Motorcraft #PW-360 waterpump.

yeah it should. are you sayin you've seen one recently and it had a 3" impeller? Cuz, at least in this part of the country, they all have 4" impellers and simply won't fit this truck. My cousin has been to numerous auto part stores and the dealer, he's looked at 50 different pumps, they all have 4" impellers.

In all honesty, I've never had a problem, so it never occured to me to measure one.