All engine blocks have a casting code. This code will tell you the year it was to be used. However, these codes were used for more than one year and could have different machining or even different main caps.
Where to find the Casting Code:
Casting codes for 1955 to 1963 are found on the passengers side of the block.
Casting codes for from 1964 to 1967 are found on the distributor pad, except the 421′s
Casting codes for from March 1967 on are found on the rear of the block just behind the number 8 cylinder