Since the 2012 Chevrolet Camaro ZL1 was introduced with 580 horsepower, rumors have been plentiful that the 2013 Ford Mustang Shelby GT500 would get a new motor to out-muscle the ZL1 with upwards of 600hp and while there are no specifications available, the new Ford-UAW contract has confirmed that the 2013 GT500 will indeed sport a new 5.8L V8 engine.