Cylinder heads used on the W-code 427 through (about) mid-to-late Jan 1968 were casting # C8AE-6090-J and are supposedly the most difficult to find.
C8OE-6090-N cast heads were used after Ford ceased using the J heads.
Intake manifold casting is C7AE-9425-F. It is cast aluminum and also known as the Police Interceptor intake due to its use on thousands of Ford/Mercury 428 Police Interceptor engines.
— Source – Royce Peterson
The most prominent date for a production GT-E engine was July, 1967. There are some known originals from August and September of ’67. Certainly once you get into ’68 all the ones I’ve seen have been service replacements, rather than regular production.
— Source – Jim Pinkerton
All 427 side oilers made after about June 1967 were drilled for hydraulic lifters.
— Source Royce Peterson