I do have a lot of books but a long way from everything Mopar known to man (or FMJ's, for that matter).
I don't know why the non-A/C 318's (or for that matter, the older big blocks with no-A/C) have a different pulley size and blade count pumps - but I suspect it might have something to do with making the plain Jane (no option) cars as cheap as they can make it, by saving a penny here and there . . . maybe. Even the plain Jane 318's with air pump got the A/C water pump (and diameter pulleys) - so i can only speculate.
The water pump (fan) pulley on my '77 318 no A/C (p/n 2951836) is between 6.827" to 6.877" in diameter.
Your water pump pulley is (p/n 3769130) is between 6.377" to 6.462" - so roughly 1/2" smaller diameter.
The smaller diameter pulley would rotate the pump about 6.4% faster - which might make up for the difference on fewer impeller blades (IDK - just taking tossing it out there to see what sticks to the wall).
The later small blocks (and earlier big blocks) finally went to one water pump for all - which should have been the choice from the beginning.
The later small blocks went to the police style water pump across the board, with larger shaft/bearings and are the ones with the different water pump housing, and I "think" they all have the same pump blade count.
The small blocks with the bigger shaft/bigger bearings have this shape:
and older pumps with smaller shaft/bearings look like this:
On a side note, water pumps work more off of centrifugal force than actually pumping. They do pump, but it seams like they create more vacuum than pressure, sometimes. Our engines have to have a coil spring inserted into the lower radiator hose - to keep the hose from collapsing. Even then, if you gun an engine, you can see the lower radiator hose shrink for a moment until the rest of water catches up.
NOTE: don't gun the engine with your head in the pathway of the fan blade! I don't want to see anyone lose their head over a freak accident.
BudW