I have direct experience with getting my rolling frame through customs when I trailered it up from Oregon.
I got to the border at way early in the morning (about 1:30 a.m.) and started workingwith the customs guy there to get it through the border and pay my tax and import fees etc. In my case, I was verylucky in that the frame still has a fiero VIN on it as there is about 3 square feet of original Fiero on the custom frame which included the VIN on the front windscreen frame.
The customs guy was going to register it into Canada as off road (race car only) and that would have put me in the position as CarBuilder says to have to get it certified with VIN, saying it was road worthy etc., I was lucky on the VIN itself as Out of Country and then out of Province at least had the VIN to start with.
He was having a terrible time filling out the paperwork for the race car route so I convinced him, and he convinced himself, to just use the regular importation paperwork saying that the Fiero with VIN was heavily modified.... I would say so since most of the Fiero was no longer part of the frame......

Anyway, I lucked out on the frame but the customs guy said if I was to go the race car route, I would have a heck of a time getting it certified in Canada at all for road use.
Not saying this is the whole story for everyone but my experience only. I will still need to do an out of country and then an out of province inspection before I get a registration slip which would be as a heavily modified Fiero and then I will need to get it appraissed for insurance so I am properly covered.
Factory Five Racing is the only one that I know in the States that has a really good explanation on how to import a partially completed kit into Canada properly and there was another fellow here on the board from Canada that had figured a way out but I have not had time to read it all yet.
Good luck to anyone who wants to bring anything more than fiberglass panels in box over the border....
Cheers
Don