The VIN on a 68 is visible through the windshield on the passenger side.
The 5th digit, the engine code, has to be a "R" to be a real 428. Being a Cali car the VIN should be "8R01Rxxxxxx".
If the guy is older and has held onto the car this long he may know exactly what he has, just keep that in mind.

__________________
1968 Mustang coupe, Acapulco Blue, 289 2v, C-4, Power Steering.
Hopefully a 1969 convertible or Sportsroof (non Mach or Boss) for next Mustang project
Trying to find my father's 1973 Mustang Grande he bought brand new. 3F04F126773 last known registration and title was in New Jersey, 1982.