each truck has to be evaluated on an individual basis, masts differ widely in design. some trucks have wearstrips or slippers to take up longitudinal play, others use tapered mast bearings installed at an angle to fit tightly in the channels and shimming takes up play on both axes. excessive play can also be caused by broken bearings or bent/spread-out mast channels. look for flaking in the channels where the bearings travel. and dont forget to stay clear of the mast components that they dont fall on you or chop your limbs off!
you need to remember that each hour of work you find can represent $70-90 of labour and that has to be balanced against the asking price of the truck and what it will be worth in safetied, ready-to-go condition. keep the big picture in mind and dont lose yourself on the details. good luck