I think Marciano would eventually find Ali over 15 rounds

[Mars_ax]

In my humble opinion the Ali/Clay who fought Ernie Terrell and Cleveland Williams would have absolutely destroyed Marciano, it wouldn't have even been competitive. Rocky would have been barely able to lay a glove on Ali because of his defense, and Mariano's short arms.

Marciano might have had better luck fighting the version of Ali after his 3 1/2 year layoff for draft evasion and/or the version of Ali who lost to Joe Frazier in their 1st fight.