From The Manufacturer:

The Challenge model is designed for the hard-core racing junkie.  The car is designed single-mindedly for racing dominance.  With the Challenge car chassis kit, you’ll be able to run with the big dogs… and do it the right way, with ultimate safety and performance.  The Challenge model is the professional competition racing version of our 65 Mk3 roadster. 

If you are a person who plans to use your car for more than an occasional open-track day, then this chassis kit was designed for you.  Built from the foundation of the proven mk3 chassis, the modified challenge car chassis meets SCCA GT1 design criteria and has been seriously upgraded in all areas to address the specific needs of the competition driver. 

Challenge cars have been dominant in racing classes across the country for more than 7 years.  Ford Racing selected a Challenge car to serve as one of the 2006 Champ Car “Fast Laps” car.  Challenge cars have earned top spots in local and national level SCCA and NASA roadracing series.