Sounds like you eliminated that possibility. That little trick sorted many issues on the old paper gasket 360 and the magnums with the plenum gaskets. Once I had a 383 Road Runner low miles that used oil. I was told no smoke but I had a friend drive while I followed. There was a haze at times but not real heavy smoke. Valve seals did not help. Back then we just did compression which was good. End result when I tore it down was the 2nd compression ring on one cylinder was in two pieces.