The 390 GT manifold was for 14 bolt heads. CJ Manifold was for 16 bolt heads. MPC indicates transition to casting with chamfer for gasket was 2/11/68 (and new H-pipe). Actual cars question that date. A 67 GT 500 with a 428 would have 14 bolt heads.
I think that's the story.
That doesn't matter. The lower two holes on the 16 bolt head, on #2 and #3 cylinders are unusable. So Ford stopped drilling them but sometimes can still be found on Service heads.
If a GT right side manifold was used on a 68-1/2 or later 428 instead of a CJ manifold there must be another reason.
I have never seen a 68 W 427 head and I assume that the exhaust bolt pattern was changed to the 14 or 16 BUT having said that, I know as fact that the 67 427 Fairlane manifolds were the old 8 bolt patterns are required access holes in the car to access them.
All FE heads cast after a certain date had the casting bosses for additonal exhaust bolts cast into them, previously cast heads, you can't modify them to the new pattern because the bosses aren't cast into the head.
Again, if this claim is true about Ford modifying the GT casting, that's a head scratchier, UNLESS it is just a supply issue and the CJ manifolds just weren't on the assembly line yet?
I'm not arguing. What is is what is, I just don't see the reason.