You said 'Mainshaft" which goes out the back.
The synchronizer hubs are anchored on that shaft. If the shaft is in the wrong spot, then so are the hubs.
the Synchronizer sleeves are anchored by the shift forks. If the hubs are in the wrong place, then so are the struts and the brass is wrong relative to the brake-cones. Sometimes the struts even pop-out when this happens, and then you have to take the cover off and one-by one, coax them back into their proper locations.
All that to say; that the Mainshaft has to be "immoveable", solidly fixed.
There are only three ways, perhaps a fourth way that the M/S can shuttle back and fourth;
1) the wrong bearing is installed
2) the bearing is beyond worn-out, and
3) the large snapring in the tailhouse is out of place
4) the small retaining snapring is AWOL; but, the rear bearing is a press-fit, so I really wouldn't consider #4.
As to #1, the rear bearing has to fit on the shaft and fill the space between the Low gear and the snap ring. And it also has to fill the bore in the tailhouse; and the bearing has to be located by the world's craziest rear bearing retaining snapring ever designed. Good luck.
My money is on #3
If I'm right, and if you have had to coax struts back into a slider, then it behooves you to make sure the strut springs are properly located. The best way is to determine correctness is to take them apart and look. But; it is possible to align all the parts without taking them apart. It's just a super pita.
If this is not done, the springs can/will rotate out of place, leaving at least one strut unsupported at one end. This will allow the slider to **** the brass and the synchronization will be crazy slow, if it occurs at all. If you force it, chances are good that the brass-ring will break.
