Every thing from K.A.R. checks out, including the inserts in the rear frame. I have talked to some Shelby buyers for more info to. I am sure it is a real GT, but it has some rare options, So I am trying to get some production figures. It is an "a" code car with 63b int. One of the things they say makes it rare is the 3 spd stick "1" code.
Review the list of GT unique items. Frame rail supports: Good thing, A code: either A or K mandatory. Pony interior: Option, not required on GT. Check the steering box, should be AW or AX. If AT, then it is a problem.
Everything sounds good so far. One caution: "Rare" is not necessarily "good". A 3 speed stick was the base transmission, an automatic or a 4 speed was optional. Therefore, typically, optional was a good thing, worth more money. A 1966 Ford Falcon 2 door station wagon 6 cylinder 3 speed stick with pink exterior and a blue interior would be rare (thankfully so), but not valuable.
Somewhat like a 6 cylinder fastback. Rare yes, but most people either upgraded to a V8 when then bought the car new, or converted them to a V8 somewhere along their life.