Pull the cap back off and note where the rotor firing prong is. Rotate the motor by hand (it's easy without the plugs in) so the rotor prong lines up with the chalk mark (also note the rotor rotation direction). You might note that when they align, the timing marks on the crankshaft pully will line up with the timing cover arrow. The bottom of the distributor has screwdriver shaped end that fits into a slot in the oil pump. You can try to rotate the oil pump shaft with a long screwdriver to align everything. Or insert the distributor so the rotor looks like it is aligned slightly ahead of the correct plug wire contact.
Somewhere along the way, someone started saying that slotted rotors couldn't be turned. This incorrect statement has become fact, or at least urban legend, so we wanted to try to clear up this misconception. If done properly, and with care, slotted and drilled rotors can be machined.