"the Mystery Motor and the Z-11 motor were identical with the exception of the 90-degree head bank angle."
Key word: "identical"
The Mk II engine block...as cast...has it's coolant inlet ports (where the water pump is bolted to the block) in a different location than the Mk I Z11 engine. The water pumps are not interchangable.
The lifter valley dams, front and back, of the Mk II are different, as cast, than the Mk I engine.
The Mk II block is not simply a W block with 90 degree head bank angle. It was a new casting.
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Rick