A lot of gray area to cover...
Was the show indoors or outdoors? (Outdoors meaning could "passerbys" see the hot rod?
Were you given any ticket or literature of what you could and could not do (as in photographing cars), or did anyone say no pictures?
If the picture is of just the car, with no people (requiring model release), you're okay. Tell the owner of the hot rod that you photographed it at a public venue, and he had NO notification anywhere around the vehicle stating a copyright on all images produced. You can stand on the street and take a picture of someone's house and sell it, and they can't do anything. Same basic principle here. You do not need permission to sell a photograph you took in a public situation, unless you were told at the time of photography, either in person or in writing that such pictures were forbidden. Tell him to consider it free advertising and don't worry about it.
If you paid to go to the car show, so much the better. Tell him to take his gripe to the promoters.