http://www.popularmechanics.com/cars/news/vintage-speed/top-nascar-engineering-cheats
read all three pages, it's worth hitting the "Skip the ad" button.
Found on http://www.dogfightmag.com/blog
If you like this kind of thing, read more about Smokey's tricks http://justacarguy.blogspot.com/2006/12/new-smokey-story.html
For a visual on how flagrant the cheating was, look at the side by side comparison of Jr Johnson's Galaxie http://justacarguy.blogspot.com/2010/01/how-junior-johnson-made-1966-ford.html
or Richard Petty's 1968 Roadrunner http://justacarguy.blogspot.com/2010/01/natl-assoc-of-what-cars-heres-another.html