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.
What you say is certainly possible but bottom line is the impression it made on you and the pure thrill in potentially owning America’s first muscle car! I’ve been fortunate enough to own several myself and now, 52 years later I find myself reminiscing about times gone by and the cool cars that were a part of those times.