Also, not sure if you are aware... but Christopher Frayling wrote an amazing book on Leone's Once Upon a Time in the West's journey to the screen.
It's a bit pricey as its still in hardcover, but it may be worth it.
https://www.amazon.com/Christopher-Frayling/dp/1909526339
This book, by the world-renowned authority on Sergio Leone, Christopher Frayling, includes revealing personal interviews with all the key players involved in the movie (in front of the camera and behind it) a wealth of never-before-published documents, designs and photographs, and the latest research into the making of a masterpiece, shot by shot.