Which Ford model initially gave the Fox-platform Mustang its larger platform?

The Ford model that initially gave the Fox-platform Mustang its larger platform was the Ford Fairmont.

Ford developed the Fox architecture in the 1970s as a flexible rear-wheel-drive platform for a new generation of smaller and lighter vehicles. The Fairmont was the first production vehicle built on the architecture, arriving for the 1978 model year before the Fox-platform Mustang appeared for 1979.

The Mustang used a shortened version of the Fairmont-related Fox structure. Its wheelbase was reduced to suit a sporty two-door body, while the Fairmont used the longer 105.5-inch wheelbase associated with compact sedans and wagons. The Mustang therefore shared the platform’s basic engineering without sharing the Fairmont’s body panels.

The Ford Granada can cause confusion because a later downsized Granada also used the Fox platform. However, it was not the model that initially established the larger Fox-platform configuration. The Fairmont and its Mercury twin, the Zephyr, came first.