The reason for that is obvious. We've preached and preached and preached about how GOOD the original factory suspension systems on these old cars are and it's true. All one has to do is look back at motorsports history and you'll see the stock style Chrysler suspensions were far superior to anything else offered at the time. These cars have won every major type of motorsports event right from the start.
Remember the article about the Movie Bullitt? They went on and on about how much modification the Mustang had to have regarding both power and handling for all of the stunts while the Charger required very little for either. It was essentially stock with a few reinforcements compared to the Mustang. The Charger still left the Mustang in the dust.
While our suspension systems can be improved upon, it's harder to do than just bolting on a few parts.
