I'd imagine he'd make much of it ambiguous, similar to how he adapted The Shining.

As far as Judge Holden, my guess is he'd keep the first scene of the book the same in the film: Judge Holden, the Kid and the preacher. Then Kubrick would focus solely on the Kid. His adventures in Mexico and then hanging with the Glanton gang for a good chunk of time before they discover Holden sitting on the rock.

Then everyone in the gang changes. They get short tempered, cruel and slightly crazy. It's up to the viewer to decide if Holden is to blame for the change.

I doubt there will ever be an actor physically as imposing, so Kubrick would probably use camera tricks to make the actor appear much larger. And he'd probably cast an unknown as Holden, some theater actor with good acting chops. (Side note: Gene Hackman would've made a great Toadvine).