Groggy must have modified his view, because later he pointed out the knocking-over-the-guy-with-crutches routine that is common to both films.
Both films feature a man who is terminally ill with tuberculosis: Victor Mature and Gabriele Ferzetti.
To me, the most obvious OUATITW reference in My Darling Clementine is the bar scene, where the gun is slid down the bar to Henry Fonda during his first meeting with Mature.
Before enumerating them, I should point out some general effects that Leone used not only in OUATITW but throughout his career. One is a particular shot of horses from the pov of a driver on a buckboard or coach; we see this used in GBU between the time Tuco and Blondie leave the mission and before they are captured by the blue bellies. We see a very similar shot in OUATITW on the drive from Flagstone to the trading post. The antecedent for these is a shot in 3:10 near the very beginning of the film when a stagecoach is held up by Glenn Ford and his gang.
I noticed something for the 1rst time in this last complete viewing, the locomotive of the train at Cattle corner with Harmonica on board is the same that brings Jill to Flagstone. Number 71.
Yep. Frank set up the meeting with Harmonica to be at Cattle Corner, didn't want to have his dirty work done close to himself if he could avoid it. So I believe that's why that set up meeting wasn't in Flagstone.
(dj and some others previously mentioned in another thread that there is a movie where Bronson played a mute Indian boy who communicated by playing harmonica, but I have not seen that movie).