What gives it away is the rear window. The Daytona was based on the '69 Charger 500 with a fastback roof style - instead of a coupe with a deep set rear window, like the usual ones - for an aerodynamic advantage. Many "Replicas" just don't have the Charger 500 rear window conversion. This one has, but the window looks plain awkward. Just examine it for a while.
The nose cone kind of looks too short, and the wind doesn't look right too. The joint of it with the body was much smoother in original ones, plus proportions are kind of strange. The badge behind the rear window is not the script "Charger" one the daytonas had. Could have been repainted, so i guess this doesn't count, but the stripes/Daytona badge look nothing like the original one's
I wouldn't know, it looked like the real deal to me at a glance when I drove by. First I noticed the long hood and nose with other stuff like small scoops on the hood. I see what your saying about the rear window, but it looks pretty flush to me, only the corner trim seemed a bit off. Do people cut out the rear window recess and weld in a fastback like this?
Yes, as far as i know, there are various kits to replicate the Charger 500/Daytona rear window. In the case of this car, as it seems like an old build, it is likely that the builder had problems with the kit, just like many bodykits won't fit well/look right on modern cars.
In the case of this car, it just looks plain weird. Most probably the fault of an amateur installing rather than being badly made itself. Compare it to a photo of a real Daytona.