The A10 & 1200 engines used a drilling in the block up to the deck to provide oil to the rockers. There is a matching hole in the gasket, & in the gasket face of the head. Another hotizontal drilling takes the oil across to a head bolt hole in the head & up to a passage that takes the oil to the rockers.

When the engine was redesigned for 1974 they simplified the method of transfering oil to the head. To do this, one of the head bolt holes was made deeper. This allowed them to drill out a small amount of the thread in that hole & send a small drilling from near the top of this head bolt hole diagonally down to the oil gallery. This brought the oil straight up to the same head bolt hole & everything was fine.
The previous drillings in the earlier heads were no longer needed & were deleted from the manufacturing process.

You have now fitted the later head, with no oil holes, to a block that feeds up to the deck & needs these drillings. Result? Oil makes it to the deck, but goes no further. Rockers run dry & eat themselves to death. Sometimes valve stems & guides suffer from the same malady if they are dry for long enough.

Actually, you would have been better off rebuilding the A10 head & using that. Same valve sizes, same port sizes, & the heater fitting hole is in the right position. [different in later heads]

Interchangeability
A10 & 1200 heads on later blocks.... YES
Late heads on A10 & 1200 blocks.... NO [unless the head is modified]