KANSAS CITY, KAN.-Nebraska Furniture Mart, the acclaimed mega-merchant founded by the late, great Rose Blumkin in 1937 and acquired by Berkshire Hathaway head Warren Buffet in 1983, is finally crossing state lines. A second electronics, furniture and appliance emporium is planned at a forthcoming 400-acre entertainment and retail development to be located near the Kansas Speedway here. The NATM dealer is pouring $55 million into the project, which will include 450,000 square feet of selling space and a 130,000-square-foot distribution center. The grand opening is slated for 2003.