We went a couple of years back in a 50's Citroen Traction Avant, max speed 55mph flat out. No good on the autoroutes so w just used the French a roads. Absolutely lovely, stopped in various villages on way down from Calais. Can't remember the exact route but your mates slow triumph could be a great excuse to see a bit more on way. We followed coast past Paris Plage (camped there on beach on way back).