Now that John F. (Jack) Smith Jr. has been elevated to chairman of General Motors Corp. (see Editorial p.7), what's next for GM's board of directors? Mr. Smith had been management's sole representative on the 15-person board for more than a year following Executive Vice President William G. Hoglund's 1994 retirement from both the corporation and the board. That management seat now is taken by Harry J. Pearce, who's elected a director and bumped up to vice chairman. Looked at another way, Mr. Pearce fills a seat vacated when outside director Paul H. O'Neill, Aluminum Co. of America chairman, resigned in November. At one time during GM's 1991-'93 crisis, Mr. O'Neill was mentioned as a possible candidate for chairman.