I have always used the stops as in my younger days i had a few 833's in which the sliders would over travel allowing the slider keys to pop out. PITA to get back in with the trans in the car, although I did become quite proficient at it. I should addthat the transmissions mare have been a bit worn and i have always driven them hard.
1. Make sure the shifter plate is not loose on the trans. If Loctite is not used they are know to come loose
2 Same for the shifter to plate bolts
3. Make sure the slots in the shifter arms are not wallowed out from being loose
4. Make sure the nuts holding the arms on are tight
5. Any chance yo might have bent a shift fork ?
keep us posted