According to Ernest Borgnine in his biography, there was a strong relationship which rose between him and Cassius Clay during the shooting. Muhammad Ali cried when he saw Borgnine's character weep during one scene.
Cassius Clay Aged 18:
How Cassius Took Rome, by Cassius Clay, Jr.: To make America the Greatest is my goal, So I beat the Russian and I beat the Pole. And for the United States was a medal of gold.
Ali is shown fighting Joe Bugner during the montage of bouts that took place between the first Frazier fight in 1971 and the first Norton fight in 1973.
However the footage is actually taken from the second fight with Bugner in 1975 instead of their first fight in 1973. Not only is this out of sequence for the montage but the second Bugner fight occurred after the Ali vs Foreman fight in 1974 which is the climax of the film.