Billboard Magazine, Feb. 10, 1968, mentioned this song, describing it thus:

Peter De Angelis Singers—WHEN WILL LOVE RULE THE WORLD (Ampco, ASCAP), (Prod. Peter De Angelis) (Writers: [Bernardo] Segall-[Earl] Shuman)—Beautiful ballad theme from "Custer of the West" film is given a smooth vocal performance by the exceptional group. ABC 11041 [b/w TWO PEOPLE]

There is a soundtrack album, described here which contains a cut labeled THEME MUSIC (WHEN WILL LOVE RULE THE WORLD).