Had to do with bolt access, yes. Mustangs, Cougars, and Fairlanes (and Merc equivalent) used these to clear shock towers and so on. I checked with Carroll Shelby Enterprises for the aluminum heads to put on my 428CJ, and was told they'd have to weld bosses to drill for the "side" bolts needed to clear the Mustang. That's the big difference with the GT and CJ heads - 14 and 16 bolt patterns for the exhaust manifolds.