Marciano by TKO. Shaver has stamina issues...but he didn't have the knack of delivering the big punch in big fights when he needed it. The Rock did.
Quarry took a quality Shavers shot, then returned heavy power, and moved in and finished Ernie off. Probably similar ending if Rocky is so-motivated.
Rocky at his peak had some decent defensive jive going on, bobbing, weaving, rolling and saw punches quite well. He'd bob and weave his way inside, taking away Shavers punching room and some of his power and punish him in there. He'd have to watch the uppercut but i could see Rocky stopping Shavers inside 7 or 8.
Shavers had was a good puncher but not hard to hit. Marciano had 1 punch ko power and good defense. I say Marciano within 4. 6 at most.