Brian,
I suspect Ron (Computerworks) will give you a more definitive answer, but I've always thought the "perky-butt" appearance was more a function of both the amount and curve of the rear fender flare on the 427, as well as the slope of the rear deck. Not having the wheels tucked in just right and higher rear height probably contributes to it as well.
But again, like everthing else, it's all a matter of personal taste. Some people like more perky and some more droopy, at least on Cobras
