andremp6 is correct. VIN numbers were stamped on both innner fender tops. I have also found that the VIN number on the driver's side isn't visible 100% of time. Even though the fender has a cut out, you may still need to move the fender away from the engine compartment to see it.
'68 Mustangs also had a dash pad VIN number plate visible from the windshield on the passenger side.
If the motor is original to the car, you can also find a VIN stamp on top of the motor just past the end of the intake manifold. It will look like this for a V8. 1968 mustang vin on motor image by mil1ion on Photobucket
I believe Marti Autoworks can also make a door data plate if you can give him a VIN number. He can also get you plenty of info on how your car was equipped from the factory for a reasonable fee.
best of luck