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You'll find the block casting code in back, in the area just below the valley where the intake and block meet, just above the joint with the bellhousing. They are generally biased to the drivers side, but on some, like the ZZ4's they are more towards the passenger side. Next you'll find a line of alpha numeric codes up front, passenger side just in front of the head. You may have to sand off paint to see them. Once you have both sets of numbers this link is one that has a lot of info - How to decode Chevy Engine Codes