Do you know why they're called "fast backs"? Aerodynamically, the fast back is faster than a notch back. This is why Chevrolet changed the Monte Carlo coupe to a "fast back". Bill Elloitt in his fast back T Bird was crushing the Monte Carlos. Back then you had to run what the factory built in NASCAR. Chevy was getting their butts kicked at the super speedways so they had to change the shape of the roof and back deck to become competitive. That's why Shelby used the fast back cars. Given every other part is identical, a fast back will go faster than a notch back.