I set up an extra MSD distributor with a steel gear for my car as a backup. Before I removed the original distributor, I set up a dial indicator and measured the distributor shaft play in and out. Can't remember the numbers now, but it was about half the amount I measured with the distributor out of the car. So far, so good. I pressed the new gear to the dimension indicated by MSD, then measured the play in the new distributor, installed it and measured again. Again, I had roughly half the play in the installed distributor.
As has been noted elsewhere, the forces on the gears act to draw the distributor shaft down into the block. The bottom of the cam gear acts as a thrust face for this "pulling down" force.
Can't recall...maybe I posted this somewhere else already...
Well...hope it helps...
Lowell