In an 2007 interview the Maestro does explain the similarities between the american music and his own describing their common sources. And the theme for
Fistful, according to him, could have been written by a troubadour.
At 2'39":
https://video.repubblica.it/spettacoli-e-cultura/morricone-intona-per-un-pugno-di-dollari-al-forum-dei-lettori-di-repubblica/363530/364087?ref=search