Another thing comes to mind, on the sides of the block, where the freeze plugs are, the 400 bulges around each cylinder.
The 2 bolt = 2 freeze plugs and 4 bolt = 3 plugs does not always hold water. I have owned a 400 that was a 4 bolt that had 2 freeze plugs. It was the one that was in my GMC when i bought it.
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