New water pump 6 impellers whats up with that!

Double M said
However, I have a Factory 1970 Dodge Challenger/Dart service manual on my desk right now, and on page 7-10, towards the bottom of the page, it says 318/340 standard pump blades 4.38" - 8, Air Conditioning 3.70" - 6.

Above is correct.

AC cars - 6 blade.
non-AC cars - 8 blade

AC cars had a pulley ratio to spin the water pump/fan faster. Both crank and water pump pulleys were different between AC and non-AC cars.