It would delusions of grandeur to think Marciano would have beaten prime Ali or Ali anywhere near his prime lol. But Maybe the plot woul thicken and Rocky would have reversed the roles and boxed Ali's ears off - not lol. As Rocky struggled against Walcott (the first time), Charles and Moore hen it is highly unlikely he would have beaten Muhammad Ali.
Here is how I see the 1950s version of Rocky against the best heavyweights seeing as Rocky wouldn't even have be a cruiserweight in the 1970s, 1980s, 1990s and 2000s lol:
Marciano vs Ali - 1964 - 67 - Ali wins inside the distance in 6 rounds or less
Marciano vs Ali - 1970-74 - Ali wins inside the distance in 6 rounds or less
Marciano vs Patterson - 1956 - 1965 - Patterson wins inside the distance in 5 rounds
Marciano vs Johansson - 1959 - 1961 - Johansson wins inside the distance in 5 rounds
Marciano vs Liston - 1962 - 1965 - Liston wins inside the distance in 3 rounds or less
Marciano vs Foreman - 1969 - 74 - Foreman wins inside the distance in 1 round
Marciano vs Foreman - 1987 - 1994 - Foreman wins inside the distance in 1 round
Marciano vs Frazier - 1971 - Frazier wins inside the distance in 4 rounds
Marciano vs Norton - 1973 - 78 - Norton wins inside the distance in 5 rounds
Marciano vs Holmes - 1978 -1984 - Holmes wins inside the distance in 3 rounds or less
Marciano vs Shavers - 1973 - 79 - Shavers wins inside the distance in 1 round
Marciano vs Quarry - 1973 - Quarry wins inside the distance in 5 rounds
Marciano vs Tyson - 1986 - 1988 - Tyson wins inside the distance in 1 round
Marciano vs Tyson - 1990 - 97 - Tyson wins inside the distance in 1 round
Marciano vs Lewis - 1992 - 2003 - Lewis wins inside the distance in 1 round
Marciano vs Holyfield - 1990 - 1997 - Holyfield wins inside the distance in 3 rounds or less
Marciano vs Bowe - 1992 - 1995 - Bowe wins inside the distance in 1 round
Marciano vs David Haye - 2009 - 2011 - Haye wins inside the distance in 1 round
Marciano vs Wladimir Klitschko - 1997 - 2011 - Wladimir Klitschko wins inside the distance in 1 round
Marciano vs Vitali - 1997 - 2011 - Viltali Klitschko wins inside the distance in 1 round
Marciano vs the Roy Jones who fought John Ruiz in 2003 - Jones wins inside the distance in 1 round
Marciano vs the super - middleweight or light - heavyweight version of Roy Jones - Jones wins inside the distance in 1 round
Marciano vs the 1987 or 1991 light - heavyweight version of Hearns - Hearns wins inside the distance in 5 rounds or less
Marciano vs the super - middleweight version of Nigel Benn - 1992 - 1996 - Benn wins inside the distance in 1 round on a good day
Marciano vs the middleweight or supermiddleweight version of Gerald McClellan - 1991 - 1995 - McClellan wins inside the distance in 1 round
Marciano vs Joe Calzaghe - 1997 - 2009 - Calzaghe wins inside the distance in 5 rounds
Yes I even think contemporary super - middleweights and light - heavyweights would have beaten Marciano badly.