Cars actually don't weigh that much compared to normal freight. I carried a load of SUV's and Mercedes-Benzes from Dubai to Almaty a few weeks back. Granted we had a bunch of regular pallets in the back and I don't remember how full the lower lobes were, but if I recall correctly were only carrying about 60 tons of payload whereas a full load is closer to 115ish. They were driving the cars around inside the airplane to position them correctly, so i guess they weren't doing anything too special in regard to batteries or fuel tanks.
I know smaller cars can be shipped easily in the lower lobe of the 747, you can even load them side ways (car's longitudinal axis perpendicular to the airplane's). That being the case, shipping one in the belly of a 767 shouldn't be too difficult.