For the Daytona 24 Hours, two Mk II models (chassis 1016 and 1047) had their engines re-badged as Mercury engines; Ford seeing a good opportunity to advertise that division of the company. The Mk III was a road-car only, of which seven were built. The car had four headlamps, the rear part of the body was expanded to make room for luggage, the 4.7-liter engine was detuned to , the shock absorbers were softened, the shift lever was moved to the center, an ashtray was added, and the car was available with the steering wheel on the left side of the car. As the Mk III looked significantly different from the racing models many customers interested in buying a GT40 for road use chose to buy a Mk I that was available from Wyer Ltd. Of the seven MK III that were produced four were left-hand drive. In an effort to develop a car with better aerodynamics (potentially resulting in superior control and speed compared to competitors), the decision was made to re-conceptualize, and redesign everything about the vehicle other than its powerful 7-liter Engine. This would end up resulting in the abandonment of the original Mk.I/Mk.II chassis. In order to bring the car into alignment with Ford’s "in house" ideology at the time, more restrictive partnerships were implemented with English firms, which resulted in the sale of Ford Advanced Vehicles, (acquired by John Wyer) ultimately leading to a new vehicle which would be slated for design by Ford's studios and produced by Ford's subsidiary Kar-Kraft under Ed Hull. Furthermore there was also a partnership with the Brunswick Aircraft Corporation for expertise on the novel use of aluminum honeycomb panels bonded together to form a lightweight, rigid "tub". The car was designated as the J-car, as it was constructed to meet the new Appendix J regulations  which were introduced by the FIA in 1966. The first J-car was completed in March 1966 and set the fastest time at the Le Mans trials that year. The tub weighed only , and the entire car weighed only , less than the Mk II. It was decided to run the Mk IIs due to their proven reliability, however, and little or no development was done on the J-car for the rest of the season. Following Le Mans, the development program for the J-car was resumed, and a second car was built. During a test session at Riverside International Raceway in August 1966 with Ken Miles driving, the car suddenly went out of control at the end of Riverside's high-speed, 1-mile-long back straight.