It would have been informative to see how Marciano reacted after being nailed by a top ten 238lbs heavyweight. We do know that 196 lbs and 188lbs heavyweights were capable of knocking him down.
Toughness, will to win are two intangibles that one would be hard pressed to find a fighter that was greater than Rocky in these regards. The one blot on Lewis record was being koed by just about single blows twice. Fighting a KO artist like Marciano this hole in Lewis armor also needs to be considered.
The point is you should never sell short an ATG fighter. ATG means the fighter has abilities far above the average fighter. Marciano showed these attributes, unlike Lewis he was never koed by anyone and he punched hard enough to KO anyone at any weight. The ability to fight low is the way to beat a much taller bigger opponent. Making oneself a small target where the larger fighter needs to lean in and down to land punches. Marciano was very adept in this style of fighting. Prime vs Prime it's not a cake walk for either man.
I would actually give the Rock a very good chance in this fight. Rocky fought out of a crouch, Lewis would have a hard time hitting Marciano and would eventually have to get lower himself (and closer). As has been seen before, ( Rahman) Lennox could get hit clean, and would not get up. He was hit with an Overhand right, and that is one of rockies best punches. Rocky would not be trying to fight from the outside, that would be suicide, he will get inside and start pounding away, Lennox will be very surprised how his extra weight , leaning on Rocky, wont slow him down. I am not sure I would favor Rocky, But by no means would he be out of this. Rocky Koed good big fighters,( Johnny Schkor was 6'4") Lennox has an additional 1 inch in height and more weight, he was also more skilled. So Rocky would not have as easy a time as he did against Johnny Schkor, whom was a solid journey man type fighter. Rocky is an ATG and better LB for LB than Lewis, I would not put him out of the fight at all. He also has the very skill set that can exploit Lennox's weaknesses.
He's ATG because he did extremely well against a bunch of mostly stiffs, mediocrities, and small aging legends. None of that adds up to much of a chance against a skilled, coordinated 6'5 245lb 84-in reach athlete who has power and agility.
It would be interesting, Lewis could hit but he lacked late power and was Ko'd 2X by good but not elite punchers. It would have been informative as well to see how Lewis would do against the elite punchers like Dempsey,Louis and Marciano if he could be nailed before he nails. There are 3 heavyweight that were great Super Heavyweights Lewis,Vitali and Vlad and they were better than the big men of the past IMO but only Vitali's chin seems to be in the very good category that being said, the size, weight of these three is a big advantage. There were no elite big men back in the day, possibly Willard and Primo being the best and Baer Simon in Louis' day. Marciano fought the 6'4 Vingo who was sub 200lbs and a few 6'5 guys but there were none in the elite class so these questions will not be answered in certainty
Vingo was 6 4 220 ( I have heard under 200 but he does not look like it) Skhor was 6 4 1/2 220 Lewis 6' 4.75" 245 Marciano would be in this, he Koed the other 2 easily.
Marciano never showed these attributes you say he had against A super heavyweight who could punch like a horse kicking. Pssst! Lewis would absolutely destroy Marciano.