Fastbacks suffer from rear lift. Their profile is an airfoil.

Need to have small flat decklid. Create rear downforce. In nascar in the late 80's when the T-bird was dominant, the GM aero coupes did not run the rear window all the way to the back. They left some decklid.

Nascar 66/67 Satellite was more successful then the pure fastback 66/67 Chargers

Go back the 1969 Car Life Magazine test. Camaro was better than Javelin and GTO.