Properly, the pinion bearing preload is determined by a crush sleeve, which is crushed down by the tightening of the pinion nut, and properly, this sleeve should be renewed every time you take the nut off, and the preload adjusted anew with the ring gear off. In actual practice, on an old one, it's probably all right to do it straighforwardly, and just tighten it a teeny bit more than it was before, to add a little more preload to the now-well-worn bearing. You should try to get a feel for how tight the bearing is before you remove the nut, and try to record how many turns it takes to get the nut off, so that when you put it back you can snug it down and set the preload by feel when you wiggle the pinion. Just remember that because this is set with a crushable sleeve, once it's crushed down you can't back up, so proceed with caution.
Obviously what I'm suggesting is not the proper way to do this, but that's how I did it on my 87, and it never gave a bit of trouble.
Make sure you have a huge pipe wrench or other device with which to hold the yoke when you loosen and tighten the nut.