Lol. Marciano shrugged off punches from ATG fighters who were also ATG punchers. Kovalev would be unconscious within 3 rounds. Marciano was a very boiled down guy with a large frame as well due to his training. Obviously the bigger man, though not "longer".
Rocky would bust his sweet "Maraschino Cherry"... get it (because it sounds like Marciano)... But no really.. Dumb question. (This could be perma.. ban worthy?)
If Kovalev is as effective at cruiser as he is at LHW, I'd pick Kovalev in his prime against Rocky in his prime. Marciano is one of the most overrated boxers of all time, all of his wins were against near 40 or 40+ year olds in the the 1940s which is like 50+ today. I'd say Bhop was in better shape and still had more left in him at 50 vs Kov than Maricano's best wins. Also people seem to fall in the trap of nostalgia. Watch Kovalev then watch Marciano, do you really think the skill level is the same? Please.
Kovalev doesn't like pressure and was eating body shot after body shot from Ward. This would be Kovalev's undoing, since Ward isn't half the fighter Marciano is and certainly doesn't have his conditioning. Marciano routinely walked through power much greater than Kovalev's.
Rocky for all the reasons outlined above and also because we know Kovalev can and has been beaten whereas Marciano was not. Therefore, there's a blueprint for beating Krusher... make him fight inside. Rocky was naturally bigger, could fight inside and strong enough to break out of the headlocks and grappling Kovalev tried to use against Ward to nullify the inside work.
Marciano would stop 50 year old Hopkins in under 10 rounds. Hop would be getting that 12th round work from the opening bell. Rocky vs Kovalev isn't a contest . Too much of an animal for Krusher who so far as proved he hasn't many levels to his game.