New York's Long Island Power Authority agreed to buy some of Long Island Lighting Co.'s assets for $2.5 billion in cash as part of Brooklyn Union Gas Co.'s proposed acquisition of the utility. The state agency also agreed to assume or refinance about $3.6 billion of Lilco's $4.5 billion in outstanding debt. . . . BellSouth Corp. agreed to acquire American Telecasting Inc.'s wireless cable operations in nine cities in Florida and Kentucky for between $67.9 million and $103.2 million. . . . Blue Cross & Blue Shield of Ohio decided not to appeal the rejection by state regulators of its controversial sale to the for-profit hospital chain Columbia/HCA Healthcare Inc. of Nashville. . . . Sears, Roebuck & Co. plans to convert its 275 Tire America and National Tire Warehouse stores into a new format called National Tire & Battery, beginning in June. . . . American Express Co. said its tax and business services subsidiary acquired the advisory, tax and business and technology units of Checkers, Simon & Rosner, which is one of Chicago's largest accounting firms. Terms were not disclosed. . . . R.R. Donnelley & Sons Co. named Emerson Electric Co. executive William Davis, 53, as chief executive and chairman.