Turn the engine to TDC on the compression stroke. Using the cap as reference, mark the distributor housing where the rotor is pointed to #1. You will install the distributor with the vacuum advance can pointed forward, but first align the rotor with #1 which is opposite at the rear. Then turn it another 45° counterclockwise to allow for the rotation from the helical gears, and install. If it doesn't seat all the way, it is probably not aligned with the
oil pump drive shaft. You'll need to rotate the crank a little until it seats.