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The MO block is a 3914678 casting with a cast date of K137 (Nov. 13, 1967)( car build date is 01D Jan 4th week) and does not appear to have been decked, however the vin on the block does not match the car.
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The datecode will work perfectly for your Z's build date. Except for the Vin# not matching, I would definately rebuild this MO engine you got with the car.