The big block Shelby Mustangs were called GT500. The first year for the big block Shelby Mustang was 1967. The 67-68 Mustang was a larger bodied car than the earlier 65-66 body style. With the introduction of the new larger body, Ford was able to join the horsepower wars of the late 60’s with a big block Mustang. In those days, the quickiest way to get more horsepower was to use a larger engine. The Shelby Mustangs one upped what Ford was offering and used the Police Interceptor 428 engine. The regular Mustangs offered a 390 as the standard FE big block engine.
This was the first year a convertible was offered. You could order the convertible as a small block GT350 or as a big block GT500.
There were actually two different big block Shelby Mustangs in 1968. The big block cars offered during the first half of the year were stock GT big block cars but with the Police Interceptor 428. These cars had two 4 barrell carbs. Mid year Ford introduced the 68 1/2 Cobra Jet Mustang. Shelby ordered all big block cars mid year on with the very special 428 CJ engine. The Cobra Jet 428 was a 428 block bottom end but with 427 heads and top end. The 1968 1/2 big block Shelby Mustang was named GT 500KR, KR for King of the Road. Where the KR comes from is another Shelby story you can read on the GT500KR page.
Ford had another big block engine, the race bred 427. It would have made Shelby sense to offer the 427 in a Shelby Mustang. But Ford and Shelby had to warranty the engines so the 428 was a better choice. It was a far more reliable engine. The ’68 order forms say that you could order a 427 engine in the Shelby Mustangs. SAAC has the records and says no 1968 Shelby Mustangs were built with a 427 engine. If the ’68 Shelby Cobra has a 427, it was installed at the dealership or afterwards. Shelby American rated the 428 GT 500 at 355 horsepower. It was closer to 400 HP.
Biff Hitzeman and I put a headliner in a club member’s black 1968 GT 500. It was an automatic. Pretty awesome car. I can appreciate why the lady owner said she was a bit afraid of the car. You could be driving on the highway at 60, kick in those two 4 bbls and squeal the tires. In those days insurance companies were surcharging cars for having too much horsepower., so car companies were under rating horsepower.
Basically, the big block Shelby Mustangs were stock, big block GT Mustangs with Shelby American add ons.