Shelby GT 500
Even though Mustang had some performance versions like the GT with the 289 HiPo V8 engine or Shelby GT 350 in 1965, the first actual performance Mustang with a big block engine and respectable 0 to 60 mph times was the 1967 Shelby GT 500. Bigger and more powerful than before, the 1967 GT 500 featured a new design, modified front and rear end, and a hefty 427 V8 engine with 335 HP and 420 lb.-ft of torque (via Hagerty).
In those days, Ford was notorious for underrating the power output of their engines, and 335 HP sounded too little for the big 427. The enthusiasts claim that the real power was closer to the 400 hp range, and the performance figures backed that claim. The 0 to 60 mph time was 6.5 seconds which was suitable for the day, and thanks to suspension modifications, the GT 500 could handle the curves well, too.