It's a long shot but you can post info about the car at the
Lost Car Registry.
In reality you may never find it without any additional info. The car may have been crushed by now. Even if the car is still around and sitting in a barn, garage, or yard there's no way of knowing, especially if it isn't registered.
Do you know the name of the person your father sold it to. You could try finding people with that name by doing an internet search then calling or writing to them asking if they owned the car and what happened to it. If you have a name with the last registration/title in 1975 then it's something to go on even how remote it may be.
__________________
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.