It looks like the Block and Heads were produced within a couple days of each other.
Block: K2561
Head: K2761
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I wonder if he got it as a Crate Motor, I never heard where it came from. My Step-Father doesn't know either because they first met in 65 or so. The Old Photo shows the Block when he got it. Can't tell if it's got Paint on it or not.
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You took a pic of the date why not the casting number too? Engines that were assembled as a unit will usually have very close dates as in your example.