The first year for the big block Shelby Mustangs was 1967. Shelby American called their big block cars GT 500. With the introduction of the new larger Mustang, Ford was able to join the horsepower wars of the late 60’s with a big block Mustang. The Shelby Mustangs one upped what Ford was offering and used the 428 engine. The regular Mustangs offered a 390 as the standard FE big block engine.
The 428 engine was offered instead of the Ford 427 for better reliability. Ford and Shelby had to warranty the engines so the 428 was a better choice. Rumor has it 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. The GT 500’s came with two four barrel carbs.
Biff Hitzeman and I put a headliner in a club member’s 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 rating cars for having too much horsepower. So certain car companies were under rating horsepower.
Basically, the big block Shelby Mustangs were stock, big block GT Mustangs with Shelby American add ons.