Not sure. I've never seen
FHOTA. I've seen scenes in the remake. I always thought that it was just a reference to a film that he admired. For me, it kind of made the Minaldi character a little more real because we see so little of him. The looming shadow character..... He's a thug and likes movies

.
I read the synopsis of
FHOTA and can see why a film about the devastation of WWI would appeal to Leone since he lived through the same with WWII. As mentioned, I would imagine that Valentino was extremely popular in Italy during his youth.
Maybe stretching it....there's a common idea about fate between the two films.
I guess in
FHOTA, there are stories of two characters returning to their native countries. In
OUATIA, it's the other side, the theme of the immigrant experience in America and being separated from the homeland.