Bob, we agree on a lot here! (Yes, I know, you don't really care about some random dude's opinion on the forum, but I'm excited that our common ground is Westerns).
I do, however, think your list is lacking two very important films and one lesser film. The two big ones are Magnificent Seven (as said before), which I think was an incredibly strong influence on Red Dead Redemption.
The next is The Big Country. Gregory Peck is an eastern with a strong sense of honor trying to get by in the rough and tumble west. It's simply fantastic and also a strong Red Dead influence. You only need to look at how the cut scenes were shot and the landscape laid out to see the connection.
Finally, maybe not one of the greatest, but certainly a favorite of mine and one that really captures the whole spirit of survival in the west is Duel At Diablo. Sure, there's a bit of a Calvary vs. Indians thing in it, but it's really about how tough it is in the west and how you have to deal with people you wouldn't because they are skilled. James Garner is amazing, Sidney Poitier is in top form (and he's always fantastic), and the Apache are really portrayed as more than faceless bad guys. It's, in my mind, the best film to capture the sense of honor and redemption that Red Dead Redemption was going for.