I've flat towed my GT for a few thousand miles to and from race track for many years.
Can't say there was any problem.
i did loose a ring gear in the diff while on the track, not sure that would have been caused by flat towing.
I was pondering this and had a thought. If the car being toed is on enough of an angle it might be possible for the front of the gearbox to run low on lubricant as it all gathers in the differential. Remember the main shaft will be turning at road speed. Interesting question.
On the GT I remove the bumper and the new "tow hooks" are welded along the bumper support boxes. They are done so that the bumper can be attached normally, just removed for towing so that the tow bar can be attached with pins.
Cost about $500 I think for installation and the tow bar.
I use tow lights that wind from the rear hatch out a driver's window, down under the hood and out the front lower grill (I have wire mesh with a cutout for the plug to feed out thru.
About 15 minutes to pull the bumper, feed the light wiring and pin in the tow bar. The lights hang out the back, held in by the hatch closing.
In the early 90s I flat towed an Alfetta race car to the races with no problems. After two years (3?) I pulled the trans axle apart for syncros and there was no wear on the ring and pinion. I was using Red Line 75/90 NS gear oil. My tow set up was the same as Paul described.
Hope this helps, G