I can't get you the exact answer you want, I can give you this info though:
There were a total of 12,087 GT fastbacks in 68. Of course not all of those were powered by 390s.
There were 6,016 fastbacks with 390s in 68, and of course not all of them were GTs.
So, I can't give you an exact answer but it's definitely less than 6,000 cars. Kevin Marti would be the only one to easily give you that info.
(Now here's the kicker, going through my books I did find that there were a total of 863 S code (390) GT hardtops in 68. Not what you wanted but I figured I'd throw it out there if anyone's curious.
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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.
Question about 1968 Mustang 390 Gt production 
