Alltel Corp. is in talks to buy 360 Communications Co. for more than $40 a share in stock, or about $4.87 billion, to expand its wireless phone business in the Southeast, sources familiar with the negotiations said. The companies expect to unveil an agreement Monday, the sources said. Both companies declined to comment. With 360, Alltel would more than triple its wireless subscribers to about 3.6 million, making it a stronger competitor in BellSouth Corp.'s fast-growing Southern region. 360 was spun off from long-distance company Sprint Corp. two years ago. The Chicago-based company offers wireless calling in 15 states, including the Carolinas and Florida. Alltel, based in Little Rock, Ark., provides local, long-distance, wireless and Internet services, mainly in rural and suburban areas in the Southeast. 360 shares jumped $5.81 to close at $35.63. Alltel shares fell $2.13 to close at $45.81. Both trade on the New York Stock Exchange.