Check the track bar first. Wander is generally the track bar or something else worn out. The track bar must be rock solid to get the best steering. Cheap route for fixing track bar is Lindsadt or Lukes link. Premium route $$ 3rd gen bracket and bar or aftermarket bar. I would recommend the Lindstadt, worked good for me, requires welding. I then went to the 3rd gen bar, also nice and no more maintenance but costly. A bad steering box will have a lot of slop. If you turn the input shaft and the output shaft turns without any noticeable play the box should be fine. Others may chime in on how to adjust steering box. People here have been recommending PSS boxes. I used a PSC box and it was also OK. I upgraded simply to get the 3 turn instead of 4 turn ratio. My box had no slop.
To answer your questions.
Take the box off with the wheels straight ahead. Make sure the new box input shaft is approximately centered between stops. Pitman arm will only go on in certain spots so should be close. To align input shaft turn key so wheel rotates freely. You can spin it from below now and it will only go on one way with the flat portions lined up. I would highly recommend the DSS to support the output shaft. These are nice trucks if the steering can be fixed. (You can mess with the caster if you want. Front must be off the ground. Mark where the eccentrics are currently pointing, on the front end of the lower trailing arms, so you can put them back if necessary. Turn them the same so the axle moves forward to get more caster. )
Attention: TDR Forum Junkies 
