I had my engine rebuilt about a year ago and it's still on the stand. In the back of my mind I think they put an AC water pump on it... I've got a non AC car. Is there a way to tell how many vanes are on the pump without taking the pump off?

From what I can remember there is no physical difference so no way to tell.

I would think 318 A/C and 340 cars would all use the 8 vane impeller water pump?

I tried sticking a wire down the different hose outlets hoping the vanes would catch the end of the wire and I could feel how many times it would catch the wire in a revolution....that didn't work. I guess I'll have to pull the pump and take a look.

I thought the casting of the neck looked different. But with all the rebuilt pumps I don't know if I would test them mixing up cores.

Or can you mix housings with blade count?

There's two different shaft diameters too.

Factory is a eight blade pump A/C 340 also got a eight blade but more often and not ended up with a 6 blade when changed over the years. Try to keep to the eight blade one on the 340 high comp as they cool that engine better the 6 is fine on the low comp one. I know it should be the other way around but I have had 340s and 360s for 40 years and this is what I have run into with them

go all the way with this: an 8 blader & on your drill press(while supporting the front shaft) press them forward until you have ~.040" clearance between the vanes & the pump body(reduces parasitic recirculation), then weld on an anticavitation plate to the vanes, then take off the timing cover & mill the front so the vanes are ~.040" from the flat, taking into consideration your particular gasket thickness and in the 2 water passages on each side there are sand casting parting lines which are slightly restrictive, deburr them,use washers to reduce your bolt length(if the thread engagement was perfect before) and space the pulley forward to maintain your previous alignment.