Always make sure all your gears fit the pinion before you install them in the cases. I've found an abundance of aftermarket parts that run a bit out of tolerance.(what else is new?) On early engines, these gears are designed to "float" on the shaft, with the movement of the flywheels on acceleration and deceleration.
Big Noot uses an "old donor pinion shaft" to apply valve lapping compound and work the gears off and on to give the gear ID the correct fit on the new shaft. Why didn't I think of that????