Testing a sure grip out of the car

You cannot accurately do it. In order to measure breakaway torque, it needs to be assembled and axles installed. There is an adapter that mounts to the wheel studs to accept a 3/4" torque wrench. With the opposite wheel on the ground, keep increasing the torque setting until you force the cones to slip. Wherever the torque wrench is set, is what the breakaway torque is.

I know some of the clutch style units approach 1000 LB FT, but I don't know any specs for the cone style.