Ali himself said that Marciano was the only champion who would might have beaten him. Hyperbole or not, the "easily" part of it is manifestly not true. Ali in a 15 round grinder.
Marciano would be KOed after being forced fed a heavy dose of jabs for 8 or 9 rounds. A prime Ali would present a formidable conundrum for the slow starting, shorter reach, and slower Marciano. This fight would be similar to Mayweather vs Gatti.
None of these things would matter. He was deceptively hard to hit because he came in low and used his small stature to his advantage. He was also a guy who could throw 90-100 punches in the 15th round of a fight. It would be very similar to Joe Frazier, and, in Ali's own opinion, Marciano was better, although I will note that I am also a huge Fraizier fan.
Ali would be difficult but a stretch to say he beats Rocky easy. Yes Ali with his gifts, talents and work ethic should be unbeatable but he lost some fights. One thing we do know is that Rock always found a way to win. This would be a battle between the grand masters of the US of A.
6'3 210lb speed demon who fought in the toughest era ever 5'10 190lb plodder who fought in a weak era Oh, that's a tough one