I would strongly recommend you rethink your plans. "May" void your warranty means that if you have any engine or transmission problems from this point forward you "may" as well consider yourself s.o.l. - and thats because you will be putting a tremendous amount of stress on your drivetrain that is all but certainly reduce your cars lifespan. 1400lbs is just too much weight for such a small car, especially over such a long distance, and there are also significant safety concerning the ability to control that kind of weight at high speeds.
Its one thing to be a little overweight on your towing and going a short distance. being 50% overweight and towing for a couple thousand miles is a very bad idea any way you look at it.