[A_Reed]

I'm not sure about keeping everything on center, but I know a way that has worked for me in the past. From my limited experience in pressing gears onto Fp's the pinion gear, if kept square(or perpendicular to the shaft), will auto center itself when being pressed on to the shaft.

So the setup I have used, consists of a v-Block with the screw clamp and a parallel. You put the Fp in the V-Block and tighten the screw clamp down onto the motor enough to hold it against the block. then place the parallel under the motor and against the block to support the shaft. when set into an arbor press everything should be square and easy to press on the pinion.