Re: Ford V-10, towing?
If you are expecting this engine to perform like a diesel, you are going to be continuously disappointed. This true because it can't match the raw torque of a diesel, nor does it develop peak torque at a point as low in the rpm band as a diesel. The 6.8 litre Ford V10 only develops 92 pounds/feet more (457 lb/ft) of torque than the 5.4, and it does it at 3,250 rpm.
In comparison, the 6.4 litre Ford Powerstroke diesel produces 650 lbs-ft of torque at only 2,000 rpm. There is a world of difference in this level of power v. what you will have in the gasoline V10.
That said, I think your problem lies in the wheels/tires that you are trying to run on your truck. A load of 6,000 pounds on an Excursion with a V10 should be a breeze, but you've pretty much killed your truck's pulling power, by making it expend an awful lot of energy driving the wheels. The 4.56 gears should help, but then putting 37" tires on it will probably kill any gain that you end up with.
I guess the question you have to ask yourself, is whether you want a cool looking truck or one that can tow well. If its looks, then big wheels/tires may achieve that. If your goal is towing capacity, you are pretty much shooting yourself in the foot with the big tires.