George Stevens Jr., director of the American Film Institute since its inception in June of 1967, was elected to the new post of co-chairman of the organization's board of trustees at a meeting yesterday in Los Angeles. The action followed the board's reluctant agreement to Stevens' request that he resign as AFI director as soon as a suitable replacement can be found.

Stevens asked for the change in order to "work on long-range AFI programs, as well as engage in outside projects." He plans to continue as the producer of such television specials as the annual AFI Life Achievement Award and the recently begun Kennedy Center Honors, and to become involved in motion picture production.

Gordon Stulberg, chairman of the executive committee, was asked to chair a search committee. Stevens said he expects to "play a role" in the selection process but has no specific candidates in mind. He anticipates the selection of a new director "certainly by Jan. 1."

The board was at pains to emphasize institutional continuity. A prepared statement quoted Stevens as saying, "At a press conference in June 1967, I was asked why I was taking the job of director. I replied, 'because it isn't there.' Now it is.

"I believe the stage is set for an orderly transition at AFI into the 1980s. As co-chairman of the board I intend to work actively to realize that future."