I am no language expert, but I believe they are actually the same word but one is French and the other is converted to English. The pronunciation of concours is French. They both mean a gathering. Concours was adapted as a gathering of horse and carriage in the 17th century. Concourse is where people gather. It's splitting hairs and neither really are correct for a authenticly restored car. But it's interesting that you brought that up. I had never noticed the two words before. And yes it's annoying when the wrong word is used. I use the wrong words all the time.
However I agree, body off is not a difficult task.