99 Sierra 2500 6.0 / 8500lb fiver

It will work, and the 6.0 having those miles is nothing if it's been taken care of. I had an 02 with the 6.0 and 4 speed tranny and pulled the same weight around with it without any issues. The engine is rock solid! However, I will say that the 4l80e only was strong as well except the TC went out on mine. When it was pulled all out and rebuilt, the clutches still looked great in the tranny and only a few were weak. The TC had cracking in in which caused it to go south...Drivetrain wise the truck should be fine, make sure that u replace the fluids. With the 4 speed you will be pulling in 3rd most of the time. Now on any steep hill u will hit 2nd gear. Don't let the RPMs freak you out. You will see 4-4500 during 2 no gear climbs. All in all I don't see any issues, if you have kept up with the truck and know it well then no worries. A lot of people have pulled that weight with there 6.0 or more.. Enjoy your vacation.

Btw my current setup works well, I pull about the same weight. But I have the newer 6 speed which works better for me in my opinion.