How much of the finished screenplay is Peckinpah's?

Robert Aldrich did a great job with this movie, but cinema junkies have long wondered how it would have turned out had "Bloody Sam" not departed following heated disagreements with the studio.

Before leaving, Peckinpah wrote and revised furiously, churning out several drafts. He got no screeplay credit, but does anybody know how much of his contributions remained intact?

"...if that was off, I'd be whoopin' your ass up and down this street." ~ an irate Tarantino