Roger, I think you took a good shot in posting your inquiry here. In the digital age, this site is a great resource for sending the ripples across the pond. In earlier posts, I've mentioned getting connected with cars from my past by word of mouth, which is how the histories of all my GTXs have been dug up. Government records seem to be a dead end for the most part because of the privacy issues. Interesting that Ontario seems to be the exception that proves the rule.
One thing I've noticed over 50 years of chasing these things, is that if a car passed through private parties back in the day, rather being traded at a dealership, there seems to be a better chance of tracking the history. An irony of my current GTX was that there was no living prior owner who knew the history of the car's first 15 years. That's because it was the dealer's personal car, and he never revealed the information when he sold the car off the lot in 1983. I filled in the history when I acquired it, took photos back in the day to prove it.