John Wayne was very disappointed that his stunt double was so obvious in the final fight with Forrest Tucker.
According to a contemporary article in the Los Angeles Times, this was John Wayne's 200th starring role in a feature film. It also noted Wayne was in the "Top Ten Money-Making Stars" list for the 20th consecutive year, and for that year he was second only to Paul Newman.
Billy tells Garrett that he rode with the worst of them, including Jess Evans and Clay Allison. Allison was a notorious gunman who worked in the New Mexico area and knew Billy Bonney. Jess Evans, played by Richard Jaeckel in this movie, led a gang in opposition to Bonney's. Unlike the depiction here, Bonney didn't kill Evans, who simply disappeared after his release from prison around 1882.
Geoffrey Deuel was nearly 27 at the time of filming. Billy the Kid was 18 at the start of the Lincoln County War in February 1878. The war lasted until 1881, when Billy was killed at the age of 21.