The MACH I was added in 1969, and was a Sportsroof (Ford's new term for a fastback...lasted 2 years). Externaly, the MACH had a side stripe, a hood scoop, and spoilers. Inside was a modified Deluxe interriorthat had woodgrain, the first high-back bucket seats for a Mustang (with a colored stripe near the top, Red or Argent), a 3 spoke steering wheel, and a special carpet. The MACH I came with any V8 offered other than the 302 and the BOSS engines.
The BOSS'es are a different story, since they are 3 different cars.
The BOSS 302 (1969-70) was a race car toned down for the street, and was the best handling Mustang of it's time. Many suspension modifications were done, and the whole purpose of this car was to go out and win in the SCCA Trans
-Am Challange. Under the hood was the mighty BOSS 302, that used cylinder heads designed for the up coming 1970 351C, and made a factory rated 290 hp (The NHRA listed a stock BOSS 302 as 335hp). The BOSSes had few options, all interriors were standard and black, Magnum 500's were an option, as well as spoilers, rear window slats, and a shaker hood scoop.
The BOSS 429 (1969-70) was an excuse to make the BOSS 429 legal in NASCAR, just less than 1000 were built in 2 years. The shock towers were moved to make room for the massive engine's cylinder heads, and the car had a hood scoop.
The BOSS 351 was built in 1971 only, and until the 2000 R-Code was released, was the quickest factory Mustang. It had a few refinements over the BOSS 302 and was built for the Trans
-Am Challange, but just as production began, Ford, GM, and MOPAR pulled all factory support from racing, and the BOSS died a premature death. The BOSS 351 engine was slightly de-tuned in 1972 and was now the 351 Cobra Jet