221 flat head engine
Your 3.6 litre, or 221 engine was known as the 81A and was a 24 stud engine manufactured from 1938 until 1942. To my knowledge it can’t be bored enough to create a 239 engine. Some 99A Mercury blocks from that era have thicker cylinder walls and can be bored quite a bit over size, but are 239 cid to start with, but are fairly rare.
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