All the 'X' is referring to in your VIN number, is what type of engine is in the car. In your case, a 4.6L SOHC (Single Over-head Cam) engine. The 1996-1998 Mustang GTs were equipped with a 6 bolt crankshaft and flywheel from the factory.
*Disclaimer* However, there is a possibility that the engine could have been replaced, rebuilt, or otherwise modified differently if you are not the original owner. Therefore POTENTIALLY having an 8 bolt crank and flywheel.