Carvair was built out of DC-4. It kept the wing and engines, but the nose was replaced with a bigger one, including nose door and cockpit up in the hump above nose door - the prototype for 747.

There are said to be a number of propeller car ferries, including Bristol Freighters.

Can widebody jetliner holds be used as car ferries? Like B747, B767, A380?

I gather that the holds are low. Maximum height of 163 cm, for all widebodies between 767 to 747.

This means that sedans can be put in holds, and so can limousines - but SUV-s and most crossovers are too high. Rolls-Royce is barely low enough.

Do passenger airlines normally offer car tickets?

They'd go as freight. Pay by the kilo (or lb). I've seen exotic cars on pallets at AMS. Seems even rather pedestrian Toyota Lexus are even air freighted. See Post 628 of this thread: http://www.flyertalk.com/forum/showthread.php?t=695692 One must be in a hurry and not care about possibly $10k for delivery to air freight a car.

Freighters have considerably more height clearance on the main deck as there's no mundane stuff as overhead bins, lights, etc. to get in the way. Not applicable to passenger flight unless you're flying on a "combi" of which further design is prohibited (the -400 combi operated by KL, BR and some others was grandfathered).

Of course if you want your vehicle to travel with you, you'll have to pay premium rates as it displaces a lot of other freight.

You would find the cost of transporting a car by air would quickly eclipse the value of most cars. We routinely transport large items by air. The last plane we chartered Texas to India pushed $1M.

Given the option of flying a $30,000 car to Europe; you would be better off buying a new car. (Couldn't do it at any price in the cargo holds of a standard passenger cabin, you would need a freighter or a combi).