Well...
http://www.riv.ca/english/US_vehicle_admissibility.pdf
Section 2..., seems to suggest that the primary way is for the car to be over 15 years old. Mentions that no kit car under 15 years old (assembled or unassembled) is admissable.
I'll be following this thread for sure..., since my future plans may make it..., uh..., pertinent.
Good Luck!