My experience
Hi, Dean.
Back in '89 I moved from Canada to the U.S., and brought a '79 Superior Cadillac transport ambulance with me. I spent less than 10 minutes at customs with this car. They checked for a sticker on the rad cradle which confirmed that this car was manufactured in the U.S., and therefore met all applicable requirements. While I was getting my paper work done, another party who was moving to the U.S. at the same point of entry tried to bring in a new vehicle that she had recently purchased in Canada. She was told that this was a "no-go."
This info is over 20 years old, so things may have changed. In addition, I wasn't exporting this vehicle for resale. You, or the buyer/importer, may have to deal with some other red tape under your circumstances.
Terry