As the distributor is installed and the gear engages the camshaft, the distributor will turn a little as the gears mesh. You'll need to account for that slight turn as the gear, the shaft and the oil pump interact to assure that the tang on the distributor shaft engages the slot in the pump drive gear. This is a very common problem. Loosen the clamp on the distributor housing so the distributor can drop into the engine far enough to mesh properly, then deal with setting the ignition timing once everything is mated and the oil pump is turning.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!