I use to think that as well but that is not right. It was running a K code 289.
I found this with a little digging at www.cjponyparts.com, “Max Balchowsky, a stuntman and self-taught race car builder, modified the 1968 Fastbacks for the film. According to the National Historic Vehicle Register, Balchowksy added quite a few changes.
Power Upgrades
390 cubic-inch big-block V8 with milled heads (built by Balchowsky)
Borg-Warner T-10 heavy-duty four-speed manual transmission
Heavy-duty Borg-Warner clutch
4:10 Positraction rear end
Heavy-duty universal joints
5-ton motor mounts
Reinforced shock mounts
Cross beam support bar
Helwig stabilizers in the front and rear
Koni shocks
Heavy-duty coil springs
Frame reinforcements
American Racing Torq Thrust mag wheels
Dunlop racing tires on the front and Firestone GP Indy Tires on the rear
Custom exhaust”
I did own a 1966 High Performance Mustang with a 271 hp 289, and it was more than a match on several occasions for 390 equipped Fairlanes.