Marciano wasn't a braggart. He was a humble soft spoken man. He said he thought Liston would have beat him. After sparring with Ali in the 1967 computerized fight, he said that Ali had the fastest hands he had ever seen but Marciano thought he could have beat him. I believe him.
I think Ali's only KOs were Liston (probably a fix) and Foreman.. He stopped a lot of people with TKOs.
OK. I wasn't being as specific to specify KO or TKO. Let's just say stoppage. By the way, who has better top-2 heavyweight KO register than Foreman and Liston?
Marciano cut easily. Ali's best chance for beating him would be to cause multiple cuts and get the fight stopped. Ali won't KO Marciano.