You are going to need to cut holes in the bodywork, and weld on steel to the chassis to provide a suitable fixing point.
You will also need to beef up the rear springs and shock absorbers, which will ruin the handling when not towing.
The tow ball will have to be set very high in relation to the height of the XK8 if the caravan is to be reasonably level.
Picturing this in my mind's eye is not pleasant.....
Having done all of that, you may still find the tail wagging the dog.
I had a Jeep Cherokee 4.0L which was downright dangerous as a tow car, due to snaking developing at anything over 50mph. I thought I was going to turn over once when it started a violent snake going downhill at 55mph: I had to accelerate out of it, which meant going up to 70mph. I was taking a huge risk doing that, but had no option. I sold the Cherokee before my next towing trip, and bought a Nissan Patrol LWB. The weight was right, the geometry was right, and it was a superb tow vehicle.
This taught me how vitally important it is to have the correct car/caravan combination: Not only my own life, but lives of my family depended on it, to say nothing of the safety of other road users.