Hello.

You are too kind.

There is no way to say 100% for sure that a 65/66 is a factory GT other than an original window sticker, build sheet, sales invoice, etc.... There are a whole bunch of ways to tell if a car is not a factory GT. And it appears that your car does not have any of those disqualifying features. It is a late enough car for the GT package to be available. It is an 'A' code car. It has the dip in the dash for the five-dial instrument cluster, etc... One thing that you could look at would be the ammeter gauge while you have the instrument cluster out. If there are two studs sticking out of the back of it,like the other gauges, that's a 66 style gauge, if it doesn't have the two studs, but instead, has a metal loop, that's the 65 style that only came with the pony interior or GT cars. In the trunk area, on the D/S, there should a rubber plug in the platform beside the gas tank. If you pull that plug out you should be able to see some reinforcement inside the frame rail that was only used on the dual exhaust cars.
Based on what you have said so far, I would say that you have a GT. Hope that helps.
