I haven't got the "Story to Screenplay" book yet, but, I did find a .pdf copy of the screenplay.
I have found a lot of changes in the screenplay adaptation by Larry McMurtry and Diana Ossana. I bought the stand-alone BbM paperback book on the last Friday of December and I saw the movie on Jan. 6.
Some of the stuff in the screenplay which is in the book is not in the movie's actual script.
In the original story, Ennis droke a truck to Joe Aguirre's office in Signal. And, while the guys are up on Brokeback, Ennis tells Jack that his brother, K.E., lives in Signal.
In the copy of the screenplay which I have where Ennis is riding with a trucker, Ennis does not even know when he has arrived at Signal.