Just installing a posi unit is easy since you won't have to remove the pinion gear. You simply remove your old unit, install ring gear on posi unit and reinstall. Make sure you install the side shims in the same side they were originally in, this will give you basically the same backlash as before. After bearing caps are torqued I usually apply Prussian blue or similar marking compound to recheck the contact pattern. I've been doing it this way for years, an have assembled about 100 or so rears without any issues.