As an american who has driven over the border a couple times, I expect a more stern inspection from Canadian than US customs.

I would speculate your chances of crossing both borders both ways without it coming up at all is greater than 50%.

As a US citizen, the right thing for me to do would be to declare the thing (as a pre 1975 exempt item) as I was leaving the country, carry that documentation with me at all times while out of the country, and present the same form when I came back into the States. I expect the Canadain regulations would be similar, but I don't know.

I would likewise expect a US customs officer to just sign the form and hand it over, and I would expect a Canadian customs officer to ask me how I could prove the thing was made before 1975. No offense. I think there are only 22 rude people in all of Canada, but they all work on the US border as immigration agents.