Yes, your Quebecois ancestors would be considered Canadian-born.

But this opportunity to seek citizenship may be time limited as it’s an interim measure in place until the government can pass legislation to amend the citizenship act to address the issues found in the Bjorkquist decision.

Your ancestors wouldn’t have birth certificates as there wasn’t civil registration of births at that time but there is a database of baptismal records (which are valid for proof of birth from that time).

That subreddit has several people who have applied based on great-great grandparents who were born in the 19th century.

Best to look at the FAQs there. The forms are on the IRCC site but the information isn’t easily navigated around the interim measure.