From what I recall, the filmmakers hired not only a dialogue coach to deal with the accents of Wyoming vs. Texas, but also the fashions that would be time and place authentic... with extra attention paid to the hats worn. Texans do not wear the same hats as those from Wyoming, and there was considerable attention paid to those things.
A lot of the rest of the fashions were likely more period authentic than anything. What frightens me is that I lived through the horrid 1970s fashions once (with the print shirts, the garish colors, the scary ensembles), and for a lot of younger folks, what they might see Jack, Ennis, Alma, and more importantly Lureen wearing is probably not too far off to what is popular to wear these days. Eeek.