Nail it down by the suffix code, which is stamped on the passenger side of the block, at the very front, just above the passenger side top water pump bolt head. You're looking for two or three letters at the end of the stamping. Go to this website, scroll down to "Suffix Portion of Chevy Engine Code" and click on the series of letters that has your letters in it.....
How to decode Chevy Engine Codes

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