General Motors Corp. has told local union officials at two factories that make its hotter-selling small and midsize cars that it will not allow any more unscheduled overtime, a move seen as a way to further cut costs. GM is burning through about $1 billion in cash per month and has promised to raise $10 billion through cost cuts and another $5 billion through asset sales and borrowing as it tries to outlast a U.S. auto sales slump that could run into 2010.Union officials at factories in Orion Township, Mich., which makes the Chevrolet Malibu and Pontiac G6 midsize cars, and Lordstown, Ohio, where GM makes the Chevrolet Cobalt and Pontiac G5, each said they were told of the overtime ban Friday.