The Greatest Story Ever Told was a movie Duke did for director George Stevens. Stevens wanted to cast Duke in the movie, but Duke was busy with another movie, but agreed to appear uncredited in a scene. Stevens casted him as a Roman Centurion in the Cross scene. He was so grateful for the favor Duke did for him. Maybe you might think that this is a movie Duke shouldn't have done, but Stevens would argue with you on that. If I recall, this movie helped Stevens propel as a major director, and he credited Duke for his appearance.
As for How The West Was Won, it seems that every western star was in that movie, and maybe it was insufficient for him to appear in the scene, but I bet it gave that movie more creditability. It may have not added much to Duke's credit, but having him appear added to the movie.
Cheers, Hondo B)