You only want around 1 1/2 to 2mm of front to back movement in the propshaft. If you have too much the drive pegs will become slightly disengaged with the pinon gears in the gearboxes and will A) badly wear the prop shaft ends, and B) could break the drive 'ears' off the pinion itself.
When I build up the transmission I mesh the crown wheels and pinions (in the gearboxes) just a little on the tight side, they then bed in perfectly on the first run, or you can run it in before with an old motor on 3 cells or similar.
G
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