Randall,
On a 1968 Mustang, the VIN is located in four places: the driver's door warranty plate, the aluminum VIN plate on the lower right side of the windshield, and on the right and left sides of the front, inner fender. The fender stampings cannot be seen unless you take the front fenders off the car.
Since the warranty plate and the aluminum VIN plate are rivited to the car and can be removed, the ONLY way to verify the VIN is to check the stamping on the inner fender, but this is a real pain. If the car has the original engine, a portion of the VIN is stamped on the upper part of the block behind the intake manifold.
Hope this helps, Russ