I won't go so far as saying his films are long, slow, and boring but I will concede his films when he was at his peak, certainly do take their sweet time. The only film of his I would call genuinely boring is Barry Lyndon.
If Kubrick was to do a Western, I imagine it'd be at the early stages of his career, maybe between The Killing and Paths of Glory, when he was still doing "genre" and hadn't really become obsessed with his "style" yet. The way I imagine it, it'd be kinda like Spartacus with its grand vistas and almost mythic portrayal of the hero.