The book this was based on is "Last Stand at Papago Wells: A Novel" by Louis L'Amour
Has the dubious distinction of three of its leading cast members dying prematurely: Tom Pittman died aged 26, Carolyn Craig at age 36, and Barbara Bates at age 43. Pittman was killed in a car crash, whilst Craig and Bates both committed suicide.
Although not lethal, the venom of the Gila monster is regarded as one of the most painful venoms produced by any reptile. In the Old West, people believed that this lizard had toxic breath and a fatal bite.
The theme from 3:10 to Yuma (1957) is used as a background in some of the quieter conversation scenes between the hero and one of the leading ladies and in the closing scene.