How do you tell what liter your Slant 6 is?

Hello
I am a recent owner of a 74 Plymouth Scamp Slant 6. My water pump has recently gone out and I need help to replace it. Unfortunately there are two different liters of the engine the 3.2 and the 3.7. How can I tell what size my engine is so that I may buy the right water pump?
Also the number and letters on my engine are as follows.
2806830-4 22031 BH
AAJ
Thanks for your help
Van Leer Mopars