If it had a 326 it was likely a Tempest re-badged as a GTO since GTO’s came with 389, 400 and then 455 as the years passed.
If that was the case, it was a masterful job. Based on what you’re saying, I’d have to say it had a 326 dropped into an original GTO. There way way too much OEM work on that thing to have gotten a re-badging - the hood, the stripes, the posi rear, the interior. It may have blown the engine and someone stabbed the most convenient mouse motor into it. I also remember him saying it had a quarter street cam in it. Sounded like a drowning hog at idle, but God coming home for dinner early after you got the tach up past 2.5K.