I think Marciano would eventually find Ali over 15 rounds

[Mars_ax]

Ali fought monster punchers from Foreman, Shavers and Lyle who hit harder than Rocky and if they could not dent the man, no one in the history could.

Henry Cooper didn't hit as hard as any of those mentioned and he certainly did dent him. Frazier did as well. The thing is, Foreman, Shavers and Lyle were all big and strong...and dumb. Just swinging those big arms until they either ko'd the other guy or exhausted themselves.

Rocky was crude and cut easy. Ali would slice him up.

I think some forget just how good Ali was in his Prime, he won the HW title from Sonny Liston when he was just 22, the muther-fucker was amazing. I seriously doubt that Marciano would have even beaten the version of Ali who fought Frazier the 1st time. Yes Frazier, who was a full fledged HW, won the 1st fight, but he took a terrible beating. I don't see Marciano who was at best a small Crusierweight, being able to get inside against Ali like Frazier did, or being able to stand up to that kind of beating. Plus, like you say, Ali would have shredded Rocky's face.