I had been looking at some 15yr+ Cobras too a few years ago, but ended up buying one already in Canada which saved me from the hassles of importing it. But there are some other members here who have successfully imported them; maybe they can chime in with their first-hand experiences.
As for the Transport Canada contacts,
HERE'S A LINK to the staff list with phone/emails etc. Looks like Harry is still there, but Alexandre is not.
I should think that any state government-issued document (such as a title or title history) that clearly establishes the car's age should work. But a copy might need to be notarized or something (
just guessing here).
But I wonder if your best bet isn't to speak with the Canada Customs office at the border crossing you intend to use, and ask
them what documentation they accept as proof. After all it's CBSA who will decide whether or not to admit the vehicle. They enforce the TC rules, and ultimately they have the authority to let the car in or turn it back.