Old pic verifying it had what appear to be small letter Polysteel 225/70/R15. That size was WS6 only that year so can help identify a WS6 car if the original wheels are gone.

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Big congrats, awesome car and paperwork!
Being a '78, it's worth trying Canada Vintage Services on for even more paperwork as besides documenting '76 and earlier Canada built/sold vehicles, they also have info available for most '77 and newer US built/sold GM product so worth a whirl, here's a link...
https://www.vintagevehicleservices.com

:beers:
~ Pete

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Thanks Pete, but this is another unique appraisal customer car I wanted to share with this community. I recently wrote a market value report for them after I inspected it. I will share here if I am notified it is going to be marketed for sale or auctioned in the future.