San Francisco's BankAmerica Corp., reaching to extend its mortgage business from coast to coast, will buy Arbor National Holdings for about $118 million in stock, or $16.35 a share. That figure provided a nasty shock to holders of Arbor securities, which closed Friday at $20. But the transaction appears to be highly likely to go through, as members of the Kaufman family tendered stock options and voting rights totalling 58% of the outstanding shares. The acquisition of Long Island-based Arbor will provide B of A an entree to the East Coast. The bank, a powerhouse in mortgage lending in California, has been on a steady march eastward this year, buying Minneapolis-based United Mortgage Corp. in March as well as a servicing center in Virginia this summer. Expansion along functional lines has been a hallmark strategy among the nation's largest banks this year.