I can think of two other things to check that were not mentioned. The hole in the dash for the fog light switch would be "D" shaped, not round. Under the rear seat there is an extra piece of sheet metal, like a reinforced patch panel, where the rear exhaust hanger was bolted in place.
Last, but not least, the wiring harness on a 65 GT is different than a non-GT. If you turn on the fog lights, the instrument and taillights will come on. All 66's were wired for fog lights, but only GT's in 65 had this feature. If the wiring harness has not been spliced into, this is one of the harder things to fake.