Hal Needham was brought in to coordinate stunts. He planned to make the wagon fly across the canyon by attaching rockets to the bottom. When Jim Brown and Fred Williamson found out, they had Needham fired from the production. But not before finishing his short acting role in the film, losing the beginning duel against Lee Van Cleef.
Second movie in 1975 where Dana Andrews plays a character who suffers a heart attack. Ironically, in both films, this is the turning point of the movie (happening at the very beginning here). The other is Airport 1975.
The Sioux City set still stands in Gran Canaria, near San Augustin.
Catherine Spaak didn't re-dub herself in the French language version of the film and her voice was provided by actress Brigitte Morisan.