Notice the differences in the 2 race cars, that Petty's was customized to race against all other "STOCK" road runners yet is obviously a ringer, the wheel wells are better, the c pillar looks smaller, looks like about an inch chop, and the rear tires look set back an couple inches, the truck looks more horizontal too.
For the Jr Johnson "yellow Banana" of fame : http://justacarguy.blogspot.com/2010/01/how-junior-johnson-made-1966-ford.html