IIRC, a stock empty diesel 4x4 Excursion weighs between 7100-7200lbs.
That's as much as a stock empty 4x4 Duramax 3500 crew cab dually...
That being said...with that much traction he will have a huge advantage, but if the driver in the Cummins knows his stuff he should be able to hold his own. The 12V also has a little less torque than the 7.3L.

I checked the numbers once, and from what I can remember that's how it played out. The Excursion is one heavy mother...and the GM dually is the lightest of the three (Ford Dodge GM) similarily equipped duallys. IFS saves a whack of weight.

 

Excursion is just a hutterite escape vechile.

 

motor wise i would have to say the cummins is deffinately the better motor. in a tug of war match, it could go either way. to many veriables like stated above. the heavier vehicle will have an advantage, the one with the best wheel traction has an advantage.

good luck, and let us know how it turns out