Interesting 1965 Formula S Info.

pishta said

author calls them both pretty close and surmised with the same gear ratio and engine they would be real close Heck the Barracuda gave 36 hp and 32 ft/lbs to the "K" mustang. Give him a 275/273 with that tachless "S". He did say he lined up the Hottest version of each car you could buy from a dealer for the contest, tell them the 275/273 was dealer available.

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The article was from early 65 and the 273-235 hp was the hottest engine you could get in a Barracuda.

Chrysler released the 273-275 hp engine that went into the D/Dart in 1966 to compete against the hipo 289 Mustangs. The first D/Dart was built and delivered on April 29, 1966. The D/Dart engine parts were available over the counter. That's the only way you could get a 275 hp Barracuda.