Not sure how its a custom, VIN decoded below. Shows it's a '71, in which case if its a GT it should have the 1 year only GT rocker emblem. Can't see that in the pictures, but you can see a "swinger" badge on the fenders, which GT's did NOT have. Also, the trunk badge is incorrect, I think that's a quarter emblem from an earlier GT. And the "GT" door emblems on the inside are from the earlier GT's, the '70/'71 GT's didn't have those either. The dash option is correct, but a most of the '71 GT's also had a console and floor shift. I thought all of them did, but looking at the GT registry spreadsheet it seems like there's a few that didn't (or didn't post complete fender tags). The rocker molding looks right, '71's had it too (you can see it on my GT's in my sig) and its part of the package for a '71 GT, but you could also get it without the GT package.
I don't think its a GT, just a '71 Dart Swinger with a few options. Doesn't appear to have a fender tag on it either, so unless the build sheet is still with the car and bears the "A65" code it would be hard to prove it was a GT. There's a few other things you could look for, namely the holes in the rockers for those '71 only GT emblems. I put up a picture of the rockers on my EL5 GT, one with the '71 only GT emblem and one showing the lack of a "swinger" badge and how the rocker looks without the GT emblem. I removed the GT emblems for safe keeping, there was only about ~1,200 GT's made in '71 and the rocker emblems are REALLY difficult to find.
As for the Dart in the ad, I'd bet its just a Swinger and not a GT. Just a car that's had some incorrect GT badges added on to it.
