[Dwight Southerland]

Something to consider is the marketing psychology of that era. It was not uncommon for the manufacturers to offer a package directed to the performance market and configure it so that it would carry some image of exclusivity. Note the SS Chevelle, GTO, Plymouth GTX, etc.which were hardtops and convertibles only. The performance engines were not available in the other model series and espcially not sedans and station wagons. The 390 engine was exclusive to the Comet Cyclone GT which was hardtop and convertible only. Same with the Fairlane GT/GTA in 1966.