It was all about shock towers.
What’s a shock tower and why was this important? Ford, who called themselves “the Total Performance Brand”, was caught napping. On the track, they were going head to head with the Mopar Hemi, on the street, their 427 big car was perceived as the be all end all in stop light performance. But the appearance  of the GTO and the rest of the GM brand’s response in 1965 was to put a 400 CID V-8 in a mid size car.

Mid size car? Ford and sister Mercury had put a performing Fairlane 500 XL and Comet Cyclone on the table, and journalists and the public ate them up. They were light, lively and crisp looking, but their source of power was the famous 289 CID V-8 in 225 and 271 HP configurations. When the GTO and later 442, GS 400, and Chevelle 396 appeared in 1965, Ford marketing knew they had a problem – 271 HP versus 335 wasn’t going to cut it with the new mid size muscle crowd.

Why not just shove the Ford 390 V-8 in the car and get on with it? Because those aforementioned “shock towers” in their unit body intruded too far into the engine compartment to fit the big Block “FE” V-8. Ford engineers and the savvy marketing people knew this because of the 1964 Ford Thunderbolt A/FX cars that had campaigned that year had to have the entire front end rebuilt by widening it almost 4″ to fit the FE 427. So in 1965, when all other manufacturers were touting 400+ CID V-8s in their mid size cars (Chrysler Corp had been doing it since 1962). Ford had no answer. Without a major redo of the unit body, Ford was stuck. Yes, the Mustang as selling like hotcakes, but in the mid size market, with Fairlane, not so much – especially when it came to what we now call “muscle”, the Fairlane was simply outclassed.

A new unit body was in the works for 1966, and the word quickly went out: “Move those shock towers!”