I just did a quick search and couldn't find that casting number on two different source websites, and also nothing on gmpartswiki.
Not disputing you read it correctly, but a little puzzled. Can you find the date code back by the starter? It would be an alpha character indicating month, a one or two digit number for the day of the month, and a single digit indicating the last digit of the year.
Maybe post a few more pictures of the engine. That might help ID it. The water pump position as mentioned above is a tell-tale sign.
