It's a hatch back hands down, because it doesn't have a truck lid, the entire back end including the rear window opens...that designates a hatch back. The fast back is the shape of the back end and still has a trunk lid.
You would have to take a coupe and somehow slope the back window down or something stupid to make a fast back, and it would look real stupid. Or you can weld up the hatch along the window and make that small little metal area underneath into a trunk lid...then you can call it a fastback.
I just call it a fox as well, nobody knows the diff between a hatch and a notch anyways. **** if I had an older mustang and it had a trunk Id want to call it a notchback instead of a fastback! I still like my hatch though.
FWIW, my Notch is classified in some circles (like the DMV) as a "sedan," and a hatchback is considered a "coupe." I think we should consult Mr. Webster for a true definition of what constitutes a sedan, coupe, hatchback, and fastback.
(I'm kind of inclined to agree that a fastback is defined as a sloping-rear-windowed 2-door coupe with a short trunk at the end. A hatchback is where the window and trunk all lift up together in a single unit, thus technically making it a 3-door instead of a 2-door.)