Matching Engine to Production Date - Pre-'68

Hello All,
I am aware that before 1968 these cars did not have a partial VIN stamped on the block to determine with certainty that the motor and car matched. However, let us say that a person was looking at a pre-68 Barracuda that had a production date on the tag as April, and the date embossed on the engine was 8*14*67, which is of course after April. What are the odds that this engine originally shipped in the car?