It could be a 262, 267, 283, 305, 307, 327, 350 or 400.

Use this list of number locations to try to chase it down further, particularly cylinder head casting numbers under the valve covers and the casting number on the back of the block at the driver's side (well, driver's side in the U.S.)....
http://www.73-87.com/chevy_ids/sbcid2.htm