He probably pants because he's hot. Dogs don't have many sweat glands and panting is their cooling mechanism.
I hope you live in a cool climate. I do animal care and rescue, and here in the hot and humid southeast I've never met an Old English Sheepdog that didn't look hot and miserable.
If you live in a warm climate, try having him clipped short. I know that one reason people are attracted to this breed is because of their beautiful coat, but a short doo is more comfortable in warm weather.
Does he seem to like riding in the car? Is it possible that he is nervous or maybe a little car sick?
Barking at passing cars: well, that's just fun, isn't it? If I was a dog, I'd definately want to do that. Have you got someone who can drive you around in your car so you can work on training? Bring plenty of treats and decide on your voice command for "stop barking". I use "quiet" but you can use anything you want. It's really a tough one because he's so excited, but it can be accomplished with patience.