Those who thought last spring's brutal film adaptation of Last Exit to Brooklyn, Hubert Selby Jr.'s portrait of America's urban underbelly, was disturbing had better brace themselves for Naked Lunch. William S. Burroughs' heroin-laced 1959 novel is being brought to the screen by creep-out director David Cronenberg (Dead Ringers). ''David and I get along very well,'' Burroughs (right) says. ''He's taken the spirit of the book and joined it with his own fantastic imagination, so it's really a hybrid.'' Plus, says Burroughs, ''He shares my interest in guns and shooting.'' The movie, which begins production in January, will star retired RoboCop Peter Weller. One thing's for sure: If the film adheres to the book's graphic, hypodermic-driven homosexual forays, it's a prime candidate for an NC-17.