The most famous version of the first Mustang iteration (1964 1/2 to 1966) is undoubtedly the Shelby GT350R, built for SCCA sports car racing, and its GT350H Hertz rent a racer brethren. But when the SCCA announced the Trans-Am sedan championship for the 1966 season, Ford faced a dilemma: the GT350R, which used the fastback body style, was homologated as two-seater. This made it ineligible for the Trans Am series, which required four seats under FIA group 2 regulations. The solution: Ford took 16 K-code coupes (i.e. the notchback, not the fastback body style) to Shelby, and had them undergo the same treatment as the fastbacks.
Receiving the same modifications (suspension changes, headers, bigger carburator, among others) meant that notchbacks where practically identical mechanically to the GT350R, but they retained their full interiors in order to adhere to group 2 regulations. They did of course have a roll bar and a racing fuel tank in the trunk. The cars were entered in the Trans Am series as Fords, not Shelbys, and won the 1966 title.
The practise of modifying a small number of notchbacks continued for the 1967 and 1968 season. It wasn’t until the 1969 season and the new Boss 302 Mustang that the Fastback body style was used in the Trans Am series. This is good news for those who want to build a Shelby replica, but don’t want to start with one of the more expensive Fastbacks. You can simply use the group 2 notchbacks as guideline, with the added benefit that the car can retain its interior. Making a ’66 notchback look like a group 2 car is actually quite simple: Shelby drop, remove the bumpers, mount American Racing wheels.