Driveshaft length tolerances +/-?

A few things to keep in mind. If you have a "monster" and whether motor plate or rubber mounts, that engine / trans can MOVE some. So allow "some extra" for bottoming. And under accel, the engine will move back and the torque will force the rear axle forward. So you need some clearance "fudge."

On the other hand you don't want it so far back that it's leveraging on the splines or part way out of the bushing.