Hello Ed,
First, welcome to The Tractor Forum. Sorry I can’t address the differences between the 850 & 860. Nor, do I know of a web site that gives the lengths of stabilizer bars.
I have a 3000 Ford. Our 3 point hitches should be very similar. My stabilizer bars are 31 ½ inches from hole center to hole center. They are the type that attach to brackets under the axle and are held on by the fender bolts. If your stabilizers are this type, here are a couple of things to try until someone comes by that is more familiar with your tractor.
Try switching the brackets for side to side. The pins in the brackets and the pins in the tractor rear end that the lift arms are attached to should point directly at each other. (my brackets have a left and right.)
After attaching an implement to the three point hitch without the stabilizers, lift the implement slightly off the ground so you can swing the implement from side to side as you try to attach the stabilizers. (I have to do this each time I “hook-up” my finish mower.)
!!Notice!! I said slightly off the ground. This is for safety reasons. The lynch pin will have to be off during this maneuver. The implement could slip off the hitch pins and fall to the ground. Don’t be caught with your foot cushioning it’s fall. (Don't ask how I know this.)
Your comment “not the external clutch” is intriguing, as I've never seen a tractor with live PTO that has an external clutch? But then there is a whole heap of things I ain't never seen. Perhaps, you meant independent PTO which would have an external means of control?