No offense but it seems to me that you should rethink the whole concept of "the ability of certain westerns to transcend the usual limitations of the genre and touch on some serious issues." It seems to me that film studies research from the past 25 years or so has demonstrated that all Westerns "touch on serious issues." You might read, for instance, Will Wright's Six Guns andSociety as a start. There is a somewhat recent bibliography in the second edition of Cawelti's Six Gun Mystique. Jane Tompkins new book West of Everything also has a few thoughts in it.