Carroll Baker, who played George Peppard's stepmother/lover, played his mother two years earlier in How the West Was Won (1962). Peppard is almost 3 years older than Baker.
The early life of Alan Ladd's character that was told in the novel was made into the Steve McQueen film Nevada Smith (1966). Coincidentally, both Ladd and McQueen died at the age of 50.
Once considered so racy it was advertised as being "for adults only", this film was re-released in 1972, and resubmitted to the MPAA for a rating. Indicating how much standards had changed in nearly a decade, it was given a PG (the PG-13 rating having not been created yet).
Filmed in 35mm, this was one of the first films to be blown up to 70mm for its premiere screening.