Pontiac vs GMC Water pumps, did not know!
 

In speaking with a rebuilder the other day, I was not aware of a difference between water pumps in the 55-62 era.
Check out the long fins on the truck pump!

Huh...I wonder if the truck pump would be an upgrade?

Coulda fooled me. I daily drove my '57 GMC (Pontiac) 347 V8 from 1990 until late 2012. Changed the water pump at least twice. It was easier to get the Pontiac part at FLAPS than the GMC part. I've used cast and stamped impellers as well as "reverse rotation" impellers (like the one on the right in your picture) which were not really reverse rotation, just a different vane design.

So I just looked in the 1960 Pontiac Master Parts Catalog and the 1960 GMC Master Parts Book. And DAMN! Pontiac used the same part number from 1955 through 1960, 540069. GMC Master Parts Book shows two different part numbers from 1955 through 1959, 518038 which was replaced by 522934.