This is correct. If you think your rear axle assy is in good condition and do not want to remove the pinion, then just install the old gear on the new spool and you can try to use the old shims, however I highly doubt they will be correct.
Once the carrier is out, I suggest turning the pinion by hand to feel for any roughness/grumble when you rotate it. If its rough or crumbly (searching for a word here to describe) then your going to need new pinion bearings. What your after when you install the spool (Note: you will need new bearings on the spool-do not try to remove & re use the carrier bearings) is correct pre load and proper back lash. The spool should install tight, and be difficult to remove (pry bar will be required when correct). Use a dial indicator and set lash @ .006-.008". 99% chance you will have to use different shims to accomplish this. The factory shims come in .002 increments. Unless you have milk crates full of 8.8 shims, you most likely will not have or be able to obtain the correct side shims. They do make adjustable or stack-able shims that will get you out of trouble. Checking pattern will be useless because you are not removing the pinion, and used gears you cannot change the pattern on. They take a set after they break in, and you will find that changing the pinion shim .010" in either direction will not change the pattern. Set lash, make sure the carrier fits tight (Note: its very easy to break the cast iron side shims when installing-coat the mating parts with a light amount of gear lube, tap lightly-pay attention. While you can check for run out (changes in the lash) there nothing you can do about it. Be careful when mating the ring gear to the spool. It should actually be pressed on. If you tighten the bolts to install it, do it evenly. You can bend the flange of the spool.
On another note, I would suggest a T/A cover.
