How to fix steering slop?

I have a 69 Dart with factory power steering. I can move the steering wheel back and forth about 4-5” without any movement in the wheels. With wheels in the air, I can see the input steering coupler move with the steering wheel however the pitman arm does not move at all within this slop range. I loosened the nut on top of the gearbox and the flathead screw was already in as tight as it would go.

Any advice on the next step would be appreciated.