The Shelby Mustang started as a stripped out version of a Ford pony car, which was basically a road racer with minimal comfort and maximum driving involvement. However, after the 1967 model year, the Shelby Mustang line evolved into a GT coupe. Bigger and more powerful than before, the 1967 GT 500 featured a new design.
Also, they modified the front and rear end, adding a big 427 V8 engine with 335 HP and 420 lb-ft of torque. But, in those days, Ford was notorious for underrating the power output of their engines and 335 HP sounded too little for the big 427.
Most car enthusiasts claim 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 good for the day. But, thanks to their suspension modifications, the GT 500 could handle the curves well, too.