Maybe you can remember back to the "not so squeezey" Fuso/Mitsubishi ads back in the day.
Both Fuso and Isuzu make, or made, a narrow body truck, but as far as I know, the wide body is the "norm".
The wide body cab should be about 1900mm/2000mm wide. Not sure about the width of the narrow body cabs, but they are tiny.
If it's any help... all the Fuso/Mitsubishi factory workshop manuals are predominantly for the FE, with small added sections specific to the FG.
To that end, most of the parts should be the same between FE and FG. I have a FG84, so I cannot help with specifics about the FG649.