There are two adjustments to the steering box, one is bearing preload, the other gearmesh. You need the linkage unhooked or wheels off the ground for these adjustments.
Start by setting the bearing preload. This is at the bottom of the steering shaft, on the out side of the box. First loosen the big locking ring using a dull screwdriver or like tool, its a counterclockwise rotation. Next screw the inner bearing cap into the box, clockwise till you feel it hit the bottom, rotate the wheel some to seat the bearings then back off just a touch, reset the lock ring. You shouldn't have up or down movement on the shaft or wheel. To check, get in the cab and pull up on the wheel, if its loose, go back and reset the bearings again.
Next setting is the gear mesh, this is the one you feel when driving, the rotational play in the wheel. On the side of the gear box closest to the engine, you will find a lock nut and slotted set screw. This adjust the sector shaft and worm gear. Start by moving the wheel to extreme right or left (I do this because the gears are in better shape out on the ends where they don't get as much use), then loosen the lock nut counterclockwise, and screw the inner set screw in. You should feel it bottom out. Leave it at that point and leave the lock nut loose, rotate the steering wheel across the center and to the opposite side. You will have to play with this a little till you get it right, if its to tight it won't return to center after a turn, to loose and you have the same problem you started with. I try to get about 1" to 1.5" of movement at the steering wheel, you said you had 3/4 turn or more!
With that much play, you might want to pull the box and check for damage, the sector shaft gears can and do wear out, the worm gears seem to be tougher and last longer. The bushings wear out to. All this wear can lead to the amount of play you describe. The ball and sockets of the draglink and tierods can and do wear.
Good luck and keep us posted on your progress, Joe.
