Replacing Water Pump

[cerebus]

I have a 71 GMC w/ a 350ci and a water pump thats leaking hardcore. Im not sure if it is the original motor. Is there a number somewhere on the block where I can find out what year it is, so I can buy the correct water pump, Id hate to buy the wrong one assuming its the original block. Or should I just remove the old one bring it in to Part Source or where ever and see if they can match it?

Any advice would be appreciated

Ps. I know that picture doesnt help much, but thats what I have to work with