I think it’s easier to explain why the head is not the real issue by explaining the reason for the bolt and plug in the first instance. So if you had say a 55-57 car or truck, and the motor went bad, bad rings, bad bearings, whatever, and you were at the dealership in say 1960, the dealer would order a service replacement block. Those fitted blocks are unusual in that they were cast with various motor mounts that would allow those block to be installed in 55-60 vehicles but those blocks were in fact the later 59-63 design blocks.
In other words, you were swapping the later design block for the earlier design block, and reusing all the other parts, including the head. So if you were swapping the new block and you were using the rest of the earlier parts, rockers and head included, you installed the bolt and plug to cut down on the oil flow to the earlier center groove rockers. If you were replacing a 59-63 block with a late design service replacement block and reusing all the other 59-63 parts, you did not install the bolt and plug.
So it’s not really the head you want to match up with the block but really the rockers. Late block and early rockers, too much oil spitting around, taking pressure and volume from the rest of the motor.
