I found a real 65 GT, there are a lot of differences. The gauge package and glove box door are different, under the rear seat 2 exhaust hangers, the fog lights also turn on the taillights, without the key on I believe. The frames tubes have been seen on non GT cars. Date code on the rear valance is nice to see, stabilizer bar is bigger, I think that if a 3/4 open end wrench does not fit over it, it's GT size. The hole for the fog light switch is notched, some might be round holes from the factory, I forget.
A borescope will tell if the hole is notched, if all of them are notched and the car you are looking at is not, then that's a flag. A paint gauge on the under rear seat hangers should show 1 to 2 mills, if it is more than aftermarket hangers might have been installed. Same with the quarter fins, if it was filled then the holes will be shown by the paint gauge.
A dealer might of converted it to a GT which was popular at the time. After a quick study of GT facts and my tools, there is no way I can be fooled by a fraud GT, and neither will you.
Remember, the factory made mistakes and had part shortages so a totality of the evidence is the proof. If the hole was missing for the switch the electrical worker would drill one.