If you have not already pulled the steering wheel you may need a gear puller or the like to get it off.
There is a seal inside the vertical column piece at the bottom, above the upper bearing. It may need to be replaced as well, mine was trashed. There is a bearing cup in the way, I used my slide hammer with a hook or L in the end and it popped right out.
I found it easier to pull it off the tractor and work on the bench but it should be doable on the tractor. You will need to detach the drag link at a minimum. The trickiest part was getting the bottom bearing to sit properly. I put the bottom bearing in, rotated the arm back so the gear inside is all the way up. With the gear on the shaft all the way up, mesh its teeth with the gear inside. Lower into the case and try to get the bottom of the shaft in the bearing, not sitting on part of it.
As for shimming the manual calls for .0118in of play (steering wheel shaft moving up and down) maximum. Mine worked out fine without having to change/add/remove shim.
Might as well adjust the steering wheel play while you are at it. Manual calls for .3937 to 1.1811in 'circumferential play'. It is done by removing the rubber plug on the right side of the steering gear case, loosen the locknut and adjusting the screw.
Charles