Rock Hudson, James Dean, Elizabeth Taylor. Some of the best-known actors of their time - of all time, really - can be seen in this week's film at the Orpheum. It isn't often these days that we have the opportunity to see longer films, and many may wish not to. But "Giant," George Stevens' sprawling adaptation of Edna Ferber's best-selling novel, successfully walks a fine line between potboiler and serious drama for its 210-minute running time. Even better, it's one of the few epic films that still will hold the moviegoers' interest.