We really need more info to give you any kind of a opinion.
Like what year is it 1976 or 2006.
Exactly what engine and what are the horse power and torque figures.
What transmission.
What rear ends and at what GVWR are they. Are they 34,000 lb tandems or 46,000 lb, or somewhere in between.
Current model and for quite a few years back a 8500 isn't really going to cut it. Like mentioned before they only have up to 300 hp and 860 lbs of torque.
Barely enough to move the truck around loaded, and definitely not what I would use to pull a 40,000 to 50,000 lb excavator on a 10,000 lb plus trailer.