The changes for the sixth generation Ford F-Series were largely cosmetic; Ford made several year-to-year changes, with significant grill redesign at 1977 1/2 and rectangular headlights being introduced for 1978 models. More substantial introductions included the addition of front disc brakes. On four-wheel drive models, the front suspension was now equipped with coil springs on the half-ton models. To increase passenger room, the cab was lengthened three inches. For those wanting a rear passenger seat in a smaller truck than a crew cab, the SuperCab was introduced in 1974.
In 1975, the F-150 was introduced in between the F-100 and the F-250; eventually, it would replace the former as the standard F-Series model. For 1978, the Bronco was redesigned; much like the contemporary Chevrolet Blazer, it was now a shortened version of the four-wheel drive F-Series.
Source: Wikipedia, 2012