Transamerica Insurance Group said that in mid-1988 it will move its 650 headquarters employees from South Olive Street downtown to a planned office tower in the Trillium complex in Woodland Hills . . . Philadelphia-based SmithKline Beckman said it will buy back up to 15 million of its outstanding common shares in an offer that could cost the pharmaceutical company up to $1.44 billion . . . The Gap increased its quarterly cash dividend on its common stock to 12.5 cents from 7 cents, payable Jan. 2 to shareholders of record Dec. 5 . . . Nevada National Bancorp signed a letter of intent to merge with Security Pacific Corp. in a deal valued at about $34 million.