Marciano would smash Wepner. Lyle would be tough but I'd favor Maricano to eventually wear him down and stop him likely after getting off the deck early. Bugner would give him a good fight and probably go the distance losing a decision. Frazier even at this stage of his career could probably beat Marciano, Marciano would have a way better shot against Frazier at this time in his career though. Marciano would have to wear him down and stop him late. Coopman goes without saying. Young would be hard for Marciano and pretty well anyone at the time he fought Ali. I'd favor Marciano to win a decision I think he'd take over late in the fight and his workrate would be too much. So basically I feel Marciano at his very best would have a difficult time doing what Ali did late and passed the prime of his career.
Due to the amount of leather Frazier was taking in Manila, it's hard not to see him getting stopped by Marciano. Equally Joe showed unbelievable grit and determination, but Marciano had dynamite in that right hand and I don't think Joe could continually take it for 15 hard rounds. On a fight for fight basis it's hard to see any of them beating Marciano, but I don't think it's that questionable to expect him to lose atleast one in a succession of fights.
Beats all except Frazier and Lyle with ease. Lyle and Frazier would be a good fight, with Marciano knocking them both out in the mid rounds. Would be better if it was a Prime Frazier, but Frazier was only in his prime for the fights he won. Ali himself said that Marciano would beat Frazier, and Frazier ranks Marciano higher than himself. Imo Jersey Joe Walcott could beat all of them except for Frazier