Not exactly sure how to find out the year----could you possibly contact the previous owner?
But it doesn't really matter as all small block water pumps fit all small blocks for the most part. There is a short version and a long version of the pump and you have to be careful about that. You most likely have the long version(along with most chevy cars), unless someone switched the pulleys (not likely). Either of these pumps will fit the engine, but it was mostly for space reasons and thus the pulleys are different for each pump.
Also, the exception is there is a reverse cooling version of the small block but i don't think you have to worry about that in your case.
Can you snap a pic of the front of the engine?