[XXXGoldL34M20]

I recently sold my LS6 70 Chevelle to a guy in California and I am in Ontario Canada. This was done November 2014 which was recent.
We both had no problems at all. Duty/Customs charged him Zero dollars at the border. As I think cars this old and being built originally in the USA they will not charge you any fees when an old American car goes from Canada back to the USA.
At both ends we had the car picked up from front door to front door and just filled out a bunch of paper work as he hired TFX Transport that I recommended. I have used them once before to do a Texas to Ontario Canada transport for me before personally and they even have a broker if needed.
Both times I had a very easy experience.

USA to Canada duty is different though as you have to pay the first Tax based on the purchase price right at the Border of Canada and have the original Title sent at a specific border crossing to be cleared as the car crosses the border which takes about 48 hours to clear.
Hope this helps.