Rocky Marciano was around 5'10" and had about a 67" reach.
Muhammad Ali was about 6'3" and had around 80" reach.
Rocky liked to slug, inside.
Ali liked to box, outside.
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So from the outset it's clear to see who has the obvious advantages and that is Muhammad Ali. He is one of, if not the best, outside fighters or "conventional" boxers of all times. He sets up combinations with the jab - he relies on speed not power - and stays outside...away from punchers and inside fighters.
But being disadvantaged is nothing new for Rocky Marciano who 9/10 is usually the smaller man in his bouts. But did this stop the Rock? Hell no. He beat a lot of great fighters in his time; Louis, Walcott, Charles and Moore. A lot of people say "well they were out of prime," but I always respond - "well what would have changed if they were younger?" In my opinion - nothing.
This being said. Facts are facts and for 13 rounds Rocky Marciano was out-boxed and HUGELY losing to a very old, out of prime Jersey Joe Walcott, and Jersey Joe hasn't got an ounce of the skills Ali had.
Ali managed to beat Joe Frazier 2/3 times and I believe Joe Frazier is a better fighter than Marciano who is tougher and faster - maybe not as strong.
Joe had better headmovement and had quicker hands and feet - and he was out-boxed and lost twice to an OLD Muhammad Ali.
If it is Prime vs Prime and a 1960's Muhammad Ali faces Rocky Marciano then it would be a one-sided massacre. Long jabs and powerful straights would wreck and cut the prone-to-bleeding Rock from Brockton and I see him being stopped on cuts, TKO or RTD from rounds 11-15 by Muhammad.
TKO13 Muhammad Ali