Great fan of Marciano, but he was the golden goose who laid the golden egg, his skin hue was favourable which made him a sellable commodity, he was portrayed as patriotic, good family man, with excellent work ethic, good manners etc etc, so any indiscretions in or outside of the ring were over looked. Even fellow boxers shielded him from criticism, such as the statement of Ezzard Charles after their second fight, when asked about the blatant fouls he suffered from Marciano his reply was, "Guess thats just way he fights."
If someone favored him over Holmes they might be able to piece something together. Ali was fading badly by say 77 and it was a pretty average era until Tyson exploded onto the scene, excepting Holmes and an aged Norton at the start. He'd be favored over Norton one would think, so Holmes would be the obvious roadblock. There's a potential 10 year window in that lot. It mostly boils down to when he might be able to beat Ali, and if he could beat early Holmes. If not it depends when he could get past Holmes, who first showed signs of slippage around 1983. Obviously it's tough at near any point in history.