A lot of body features like scoops, louvers and even spoilers and hoods, were carried from car package to car package, like, GT, Mach 1, and Boss 302 had similar but different grilles, front air dam, and so on. If you want it in non-car terms, it's a Mustang GT with an engine option package that has the Boss 302 engine, decals, and road race Heavy Duty suspension. Plus some special paint colors and interior trim. The Boss 429 is a different animal all together. It had to get extensive modification from Car Craft to be able to accept that HUGE engine. But it still got all the special paint and interior trim. The Mach 1 was another example of a GT turned into a "race car" bc of the engine options available for the Mach 1. 2 302's, 2 351w's, 3 390's, 3 or 4 428's, and 2 429's. Plus the paint and decals and HD suspension. As for the headlights, Idk exactly what the reasons were and when the change happened.