STS or Radix would be the way to go. I pull my 8500# 5th wheel proubly 9500# with all the crap in it and pull 70-75 mph with no problems. I added a small IC that keeps the IAT's down when under boost, which it only see's boost when taking off or going up steep hills and bridges. At 70- 75 boost gauge at 0 boost and it never get out of OD and get 14-14.5 mpg towing. Lets just say i'm one happy camper

 

yep...pulled in over 8,000# of hunting stuff off the lease today and averaged 14.0...no complaints here either

 

i deffinitly agree with the radix being a great choice for your truck. this is the reason i think the 6.0 in an HD is a terrible choice if you plan on doing any heavy pulling. they are a great workhorse in the way that they will run under heavy throttle all day long and not be bothered by it. the downside is that with only 364 cubes you'll need to. now a 6.0 with boost, it should pull right up there with an 8.1.