I'm a little confuzzled. I had a '65 notchback that had what appeared to be factory rear lapbelts. It also had factory front discs and a 4-speed.....I hope it wasn't a GT.
Seat belts were optional in 65, or at least late 65. The late 65 cars had the holes for the rear belts pre-mounted, even if the car wasn't optioned as such. One panel fit all.
The GT package was strictly a performance and outside appearance group--something more than a normal car and yet less than a Shelby.
Disc brakes and 4 speed are not really rare options--just checks on an order sheet. It takes a lot more than that to make a GT.
65/66 GT's had heavy duty suspension with a larger front sway bar, fast ratio steering, disc brakes, dual exhaust with trumpets that exited out holes in the rear valence, side strips, fog lights, M-U-S-T-A-N-G spelled out with individual letters in those side strips, and a 66' type gauge panel which includes an amp guage. A 4289 with a 4 bbl carb was also mandatory (A or K code.) An A code would probably have a 3.00 rear gear ratio, which is part of its option package.
GT's could be 3 speeds, 4 speeds, or automatics. Luxury (pony) interiors were not part of the GT group, but a GT (or any Mustang) could be ordered as such. No special steering wheel or console either. Any of these could be checked on an order sheet. Many of the GT features could be checked too and yet you wouldn't have GT.
Another way to get full gauges in 65 was to order the Luxury interior, which was a more common option than the GT equipment group.
The best way to ID a true GT is by checking the exhaust. It should have the supports mentioned on the rear floor panel and it will have special support found inside the rear frame rails to support the rear of the exhaust.