NCRS references say 3914678 is the correct casting number for a 1968 Corvette 327, and the C 5 8 casting date says it was cast at the Saginaw Foundry on March 5, 1968; the partial VIN, 18S414180, says it was originally installed in a 1968 Corvette built on March 20th, and the 300hp engine was assembled at Flint V-8 on March 7th.
The original cylinder heads were 3917291, and their casting date will appear directly below the casting number, as shown in the photo below.