In my opinion, there are many other things to look at other than a buck tag for GT identification. As mentioned, the exhaust reinforcements are a big one. Also, a GT should have a 5 dial instrument cluster, so if the car has standard interior, it has to have the dip in the dash structure for the cluster to clear.
Also, have found 'Addressograph' stamping on original buck tags. Very faint, but very cool when seen. If too much paint applied, easy to cover up.