Word of advice. Don't trust anything that the government tells you. ;-)
I recently built a GTM for a Canadian customer. He built the car to go-kart stage and then hauled it down to me personally to have me do all of the bodywork and paint. When crossing the border, he told the Customs agents exactly what he intended to do (bring the car to me, have me do bodywork/paint and then bring it back into Canada), and the border agents assured him that it would be no problem to do that. I did my job and he came back to pick it up, headed home and his GTM never did make it back across the border. He ended up sleeping there at the border while they tried to figure out what to do.....finally had to leave it sitting at the border in the car trailer and head home. Never did get it across the border.