At one point in Giant, the epic 1956 film about the wealthy Benedict ranching dynasty in far West Texas, the family’s youngest daughter, Luz Benedict II, vows to fly to Dallas and go to Neiman Marcus to buy a “starlight-white evening dress” that’s “very plain and simple and deadly.”So when Vicki Chapman chaired Dallas’s Cattle Baron’s Ball—a lavish annual fundraiser for the American Cancer Society—on the occasion of the movie’s fortieth anniversary, she thought it only fitting that Neiman’s agreed to cohost the world premiere of a newly restored version of the film as the kickoff to the Western-themed event.On the Friday night before Saturday’s hoedown at a ranch in Kaufman County, Giant cast members, including Jane Withers (who played Vashti Snythe) and Earl Holliman…