I think Marciano would eventually find Ali over 15 rounds

[Mars_ax]

Height, reach, weight and speed are all in favour of Ali. What Smokin Joe had in his favour was continuous head movement as opposed to Marciano who just kept coming at you with that crouching style, but he didn't have that awkwardness that Joe Frazier had. Marciano would've been a lot easier for Ali to hit than Smokin' Joe. Marciano was a big puncher, but if Smokin' Joe couldn't knock Ali out then i doubt Marciano would've either. He never fought above 190lbs as World Heavyweight Champion. Ali at his peak in 1966/67 was at 212lbs and was heavier,taller and faster with a long reach that would've seen him stabbing that left jab in Marciano's face all night cutting him to ribbons by the mid the rounds. Marciano only fought 7 World Title fights including 2 each with Ezzard Charles and JJW. It's only a 3 year reign at the top. One of the things that surprised Liston when they entered the ring was Ali's size, he was a big man.

Good point, Rocky had only 7 world title fights in his 3 year reign as HW Champion: 2 against an aging Journeyman, Joe Walcott, 2 against Ezzard Charles, another Journeyman Crusierweight, who ended his career with 25 losses on his resume, 7 by KO. And a who's who of chumps, Roland LaStarza & Don Cockell, finishing his inglorious reign against Archie Moore an ancient, blown-up light-heavyweight who knocked him on his ass.

Now let's look at some of the pugs Ali faced in title fights: Liston, Patterson, Terrell, Frazier, Norton, Foreman, et al. In re: the level of competition the 2 faced in Championship fights it's no contest, Ali's was far superior, plus Ali had an extensive amateur background, winning a Light Heavyweight gold medal in the 1960 Summer Olympics in Rome, his amateur record was over 100 wins. Marciano's amateur record is almost non-existent.