Looking at getting a new water pump but I've heard getting one with a cast aluminum impeller is the way to go as it does not erode in the coolant.

I checked rock auto but none of them specify which material the impeller is made of.  Any recommendations on a pump?

You might be thinking of stamped steel versus cast iron.  I've never seen an aluminum impeller.  

Compare the AISIN and Airtex pictures to the others. I think that most people like the cast impellers.  They were hard to find for a few years but it looks like they're back.

https://www.rockauto.com/en/catalog/nissan,1978,280z,2.8l+l6,1209260,cooling+system,water+pump,2208

 

so Aisin would be the way to go and shouldn't rust?

Your antifreeze should contain corrosion-inhibiting chemicals.  The block itself will rust if you don't use proper antifreeze, it's cast iron.

Don't forget the washers on the pumps. Lock washer against the bolt head then flat washer against the aluminum pump.

16 hours ago, Zed Head said:

most people like the cast impellers.

Yes!..  the best pump, not that cheap sheet metal impeller.

Go with the Aisin WPN013, cast impeller, its a very good pump.  Its available from Rock Auto or Amazon.   About $25