An old rule of th thumb I learned years ago before the current advent of gcwr ratings. 2x grawr = max trailer at you want to pull with a given rig. The more hp torque, gears etc, = better performance.
If your truck has say 4000 lbs of tires as lowest denominator tween Adele, springs, braking capacity etc to range, then max trailer is 8000 lbs you want behind the truck.
Generally speaking. For older half tons like this, 5-6000 lbs is probably about max you want to pull from a chassis standpoint. You may have the engine that can do 10-15k......go for lowest denominator.
Marty