As far as the solid pinion spacer--it is an aftermarket part offered only for the 489 case to eliminate the crush sleeve (the pinion nut can back off, with the crush sleeve). The 741 and 742 cases use pinion bearing preload shims--and they have nothing to do with pinion depth. You can swap the pinion yoke without worry. Just remember to replace the pinion seal, torque the nut to specified torque, and use red locktite on the pinion nut threads.
Hope this helps some.