8 vanes = non A/C pullys-turns slower and pumps x amount of water at idle.

6 vanes = A/C pullys- turns faster and pumps same x amount of water at idle.

at higher RPM with the 8 vanes/smaller A/C pullys there is a chance it will cavitate.

the t-stat by-pass hose should not be blocked, it keeps the vanes from cavitating while t-stat is closed. you can block it if you run a restrictor plate instead of the t-stat.

welding a anti-cavitation plate onto the vanes helps flow and prevents cavatation.

is this the early cast iron WP?

I get a new, heavy duty cooling 5/8 shaft/large bearing 8 vane pumps at napa for 65$ (neck has ribs not gun barrels)

with all that said I have mixed match vane count and pully size many time and never had any trouble as long as the rest of cooling system was up to specs.

put a 6 vane on and run it.