Sounds like you're going to be real close on the numbers, especially after you load up all your stuff. A suggestion would be to jump up to 265 tires on the truck (if you haven't already), as that will give you almost 900 more lbs of tire capacity, which is a weak link, IMO, on that model. Another thing, as I'm sure you've found out, is that the brilliant people at GM decided that we were too stupid to know how to use an OD cut-out switch, so you go from OD to 3rd when you downshift manually. Even in T/H mode, it can be hard to get it to drop down. I'm considering the BD Power Tap Shift, which gives you fully electronic shifting on the column =and= allows you to use 4th manually. Have fun!
Lyle