Director of photography Mario Bava was called to step in as director (uncredited) to finish the film, as Jacques Tourneur did not wish to extend his ten-month contract. Galatea Film decided to reward Bava by giving him a chance to develop his own project as a solo director, Le masque du démon (1960).
It's pretty clear from the title on the screen that it translates to "The Battle of Marathon", which makes more sense since a good part of the film is dedicated to that particular event, even if it is greatly fictionalized. The change to "The Giant of Marathon" seems to be focused on highlighting , Steve Reeves, which also makes sense.
Mylène Demongeot says in her biography that Steve Reeves, despite his impressive muscles, was not strong at all--he hardly could carry her on his shoulders.
The same year, before or after the shooting of this film, Steve Reeves had a serious shoulder injury whilst he was on the set of Les Derniers Jours de Pompéi (1959).
According to Mylène Demongeot in her biography, Steve Reeves made movies only to earn enough money to buy horses and a ranch.