For the cost of a pump, unless you have low miles on your pump, I'd get a new one. The Series I, II and III pumps are interchangeable. Some notable facts:
Series I - Has a good cast impeller that promote better coolant flow. To use this on a Series II, you must also have a Series I pulley (the diameter is the same as the stock pulley on the Series II, but the center bolting area is different).
Series II - Almost all factory pumps (if not all) had crappy stamped steel impellers. Sure, they got the job done, but they could be better.
Series III (not all) - This has the same cast impeller design found on the Series I pump, but works with a Series II pulley.

I actually did a Series III pump upgrade on my Monte. It was a convenience issue. I was swapping to aluminum pulleys on my alt, tensioner, water pump and power steering. I had to pull the power steering pump, which gave me easy access to the last bolt or two on the water pump. Figured at 70+ miles, spend the about $30-$40 and put a new pump on (with an upgraded design). Works great, happy I did it.

 

Did not know the impeller was changed like that. Good info there! I never searched into them much because I went with a meziere electric water pump

 

That is great info. Didn't realize the Series I could be used with slight modification. All these years messing with 3800s (probably know more about them than I do SBCs) and I still learn something new. I love it.

 

Yeah, I learned that info about the pumps from the Bonneville guys. Not a bad mod if you have to change a water pump.