Picture 81 (the first one) shows the left hand arm, and its positioning feedback adjustment. You may be able to adjust it to limit the amount you can move the arm up. It will not stop the arms from being ground-driven, and smashing into your cab.
Picture 80 (the second one) shows the right hand arm. You can remove the circlip and pull the arm off the splined shaft, rotate it a notch or two down, and slide it back on. Do the same on the left side by removing the feedback assy - the two center bolts - slide off the left arm and remount it the same as the right one. The raising of the arms will then be limited by the internal 3PH cylinder.
Moving the arms down a notch or two also lowers their downward travel, so be careful if you leave it all the way low, and back up - they can catch on things, with less than desirable results.