2OO2 HD2500 4X4 8.1 GAS/ ALLISON TRANNY

I bought the truck with intentions of pulling alot of trailers and have since just used it for highway miles from san antonio to austin daily. It has 170k and i plan on keeping the truck for another 170k and was going to pull everything out at one time (got a deal on a 6.0l engine/tranny/transfer case with 8k miles no purchased yet). Have intentions of saving some fuel and money on replacement part prices for the future.

Not like the good ol days when you changed a few mounts and drive shaft lol.

any help would be nice  

I question the logic of this swap. The 8.1 is built off the old proven Big Block "Rat" motor that goes back about 40 years and parts for it will likely never be hard to fine for the foreseeable future. 170K is nothing for a well maintained big block plus you have the allison tranny which is strong too with a tall OD (.69 vs .75 on 4L80 used with 6.0 and also the ally has a 3.1 first gear vs 2.48 on 4L80) I think the best bet here for you is to use stock type tires with smooth tread (something like a 235/85-16) to minimize drag and consider gearing truck taller not deeper. A 3.42 axle ratio with no lift and tires above would yeild a little over 1600 RPMs at 65 MPH and unlike a 6.0 the 8.1 would have the torque to effectively handle it. Do this and little tweaking  and use 89 octane or better (that engine LOVES higher octane)and you should be able to get in the 16 to 17 MPG range at 65 MPH. It is not that 6.0 is a bad motor but you have a great motor and tranny and I would work with it. One more thing, even with a 3.42 axle ration that 8.1 would beat the pants of  same truck with a 4.10 and a 6.0 when you put you foot into it. If you are real serious about MPG, no lift and no big tires.