DZ: Yes, you nailed the Mystery Motor. I was trying to stick with "available in a production car" big blocks. And you're right about the Smokester, he liked the Mystery motor, but dislikes the resulting Mk IV. (I don't understand why)
As I understand it, the Mk III was to be the Packard V-8, Chevy was to buy the tooling from the bankrupt Studebaker-Packard Corp, but the deal fell through.
Think about it...The Packard engine has 5" bore centers, while all Chevy big blocks have only 4.8 bore centers. If updated to today's casting technology, and siamesed, we'd have 4.75 bores without too much problem! In fact, the Packard engine wouldn't be legal for NHRA because the bore spacing is too wide. No doubt Chevy would have had to toss the Packard heads, though.