The FSM is your best friend:
http://www.nicoclub.com/FSM/J30/
I would strongly recommend purchasing a Nissan OEM remanufactured or new alternator. In the many years, I've been on this forum, only one member(out of the several hundreds that have come through) has had good luck with an aftermarket alternator. Aftermarket alternators are well known for causing all sorts of electrical, idle and ignition issues on the J, so avoid the headache.