Have never used this company, but it looks like they have the Deluxe GM belt (as well as other styles) in all the sizes and colours. Plus any accessories you might need.
The price looks fair, now mind you it's US dollars, but if you could ship to a border town close by then it might be not too bad. Says $5 shipping in the USA. I plan on buying 6 standard belts from them.
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65 Laurentian post, 67 Grande Parisienne 4 door HT.
If repro is not what you seek, try Ssnake Oyl for restored original seat belts. Here's a set for a '67 GTO that would be the correct deluxe belt set for you: Restored 67 A39 Deluxe Belts
At $749 USD that is pretty rich for mortals. It depends how nice you want them and how independently wealthy you are. Jeez, they were a $9 option in '67.
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67 Chevelle Malibu Sport Coupe, Oshawa-built 250 PG never disturbed.
In garage, 296 cid inline six & TH350...
Cam, Toronto.
I don't judge a man by how far he's fallen, but by how far back he bounces - Patton