Just put mine on my 73 cuda, I installed the arms first, loose. Then the bumper and inner plate to the arms, snug. Then positioned the bumper front to back side to side ( not a lot you can do side to side without shims or moving the bumper on the arms but not much room there either)until it measured right (get a second set of hands) and tightened the arms to the frame.
Admittedly I had practice when I was doing the body work mock up.