The Cummins/G56 combo is derated in HP/torque and tow ratings because the limiting factor is the clutch, especially for an inexperienced driver. There are plenty of good aftermarket solutions to that which I will take advantage of when the time comes. I may consider a mild tune at that time too, but the truck tows just fine in stock form. And I'm not towing a 8-9k lbs TT; it is a high profile 16k triple slide 4 season fiver.
The Cummins has such great low end torque that it makes it a great choice for the manual transmission and really makes one appreciate the capability of the engine.
Oh, and one last thing before I opt out of the discussion; in my area you had better be prepared to jump on a cherry Cummins/manual tranny truck quickly and pay premium price for it because they sell within a day or two. And that goes for all years of them. The 7.3L/ZF6 Fords are the same.
OP, if you decide to sell your truck you will have no problem at all and you will get top dollar for it. Just be sure that you want to sell it before listing because you will have people breaking the door down for it! :B