Ali was past his prime when he met Frazier in 1971 so I don't think that bout is a fair litmus test for a hypothetical fight with Marciano. The prime Ali of 1966-67' wins hands down. The slower footed Ali of the 1970's would be an interesting match-up.
depends on the constant pressure, ali had trouble with pople who came head on and never stoped, ali frazier I and III, could rocky hurt ali like frazier did in manila, if joe had went out for the 15th i think ali would of quit on his stool or been koed/tkoed. but good fighters get the rub of the green, ali who stuck and moved could do a joe walcott rocky marciano I without the ko at the end. difficult to sat rocky never fought a big guy so its hard to say, where was ali did fight some pressure swarmers.