that 104 is quite a challenge !
thankfully, the Mustang is easier
coupe : http://images.gtcarlot.com/pictures/39676218.jpg
http://images.gtcarlot.com/pictures/39676226.jpg

hatch : http://images.gtcarlot.com/pictures/6444798.jpg
http://images.gtcarlot.com/pictures/6444808.jpg

Also, there was no GT coupe, they all were either hatches or convertibles ( except for 1982 when Ford re-introduced the GT but did not officially had the convertible back yet, it came for 1983 )
SVO ( 1984-1986 ) and Cobra ( 1993 ) are also hatches only

i'm not too familiar with the 1974-1978 gen so i'll have a closer look before changing anything

Similar goes for the Celica and Nissan SX as night cub suggested ( both, if i remember offered all the versions in both styles )

There is also a possiblity that most of the Celica are already classified since they had the Liftback name in extra field

Last, i won't change any that does not offer any visual difference ( big close-up of the front for example )

How will you do the change? Visually is it easily possible to tell them apart?

This makes me thing to the Peugeot 104, launched as a 4-door sedan:
https://en.wikipedia.org/wiki/File:Peugeot_104_1974.jpg
and changed to hatchback in 1976:
https://en.wikipedia.org/wiki/File:Peugeot_104_GL_(15436439521).jpg
Not easy to tell them apart

i agree . If no objection i'll start slowly with the Mustang

I think it would be useful to separate them for models that offer more than one two-door bodystyle, like the Mustang, Celica and Nissan SX models. For cars like the Probe, 3rd-4th-gen Camaro/Firebird or Scirocco, it's not as much of a concern.

The only one where it would get tricky would be the Geo Storm, which had two different 3-door models. I guess also the AMC Spirit/Eagle SX4 also had an issue with the slopeback and kammback versions.

I think it would depend on how it was classified in the brochures ? No ?

I know night cub did some research in the past on other models 2 door sedan vs 2 door coupe. He might have some insight?

While exploring the Mustang case i saw that a lot of hatch ones are classified as coupe ( might be same for Celica ) and some others as hatchback. I do understand why we get for those two cars a different classification depending of the model but should we fix all the hatch Mustang ( and possibly Celica ) as hatch ?

Speaking of the case , i suppose that Integra, Probe, Tiburon and similar hatch-opening coupes are classified as coupe due to the shape of the rear , that goes on a slope instead of closing straight like a VW Golf for example, am i right ?