I don't know why you are calling it a "kick back bracket", it is simply part of the cutterbar lift mechanism.
2 things... first you need to swing the drawbar to the side so it is NOT directly under the PTO shaft, then you need to set up the lower chain so it holds the mower frame the correct distance above the ground and stops excessive lowering of the 3-point.
THEN you set up the upper chain so the cutterbar lift works correctly as the 3-point rises.
You REALLY need a manual, as it details all these steps.
All that being said, I hope it's not a nine foot mower set in the wide position, because I have one on an IH 424, a tractor twice as big, and that's plenty of mass flopping around behind my bigger tractor.