Bigger engine doesn't always mean more tow/haul capabilty, its better suspension, tires and gearing. My uncle hated his 8.1 for towing his bobcats and other small loaders, his claim was the 6.0 had better usuable power. You would be better off just doing some of the engine upgrades previously mentioned, also getting a shift kit at the least foe your tranny, letas also not forget the 8.1 won't net any better mpg then your 6.0. Also weight distribution of what your towing will come into play, along with the type of trailer you are pulling single or tandem axle, gooseneck/fifth wheel. Also your tongue weight.