Yes. If you install new bearings, and then recheck your settings and they're out of spec, you can add/remove shims as required to get your settings back into spec. That's what shims are for. Getting the specs back within tolerances will keep your gears from being damaged.
The only issue you may have is getting a super-good wear pattern mark on the used gears because of the fact they're already worn in. You can do it, it's just not as obvious so you have to look more carefully. And I'd worry more about the coast side pattern on used gears as they're probably not worn in to match each other as much.
Just a few things you have to remember and some tips to make things easier. (I'm not an expert but if you've ever done a rear gear setup, you remember a lot, especially if you've had to disassemble that thing a few times to get it right.)...
Remember to REMOVE the rear axles first. You'd be surprised that some people get into it part way and then remember the axles are still in the way.
Be careful with the pinion cross shaft retaining pin. May be a good idea to have a new one handy anyway as your old one is likely broken inside, but you won't know until you start to remove the old one. This simple little bad idea of a bolt has ruined a lot of good days under the car.
Make sure to use proper gear marker paste. There's a reason they make it in bright colors. So you can see the pattern easier. Don't use prussian blue or some other crap, your pattern is likely going to be hard enough to read on used gears.
Rule of thumb- when removing a properly shimmed rear carrier, you must use a pry bar of some sort to get it loose. Don't drop it. But if it comes out by hand, it wasn't properly shimmed.
Shims/spacers- GM used cast iron side bearing spacers from the factory, I'm not sure what the new spacers are made of. The CSM says to use new service shims/spacers when redoing a rear end. I've reused the cast iron spacers without issue. The worst they can do is crack while installed, but even if they crack, where they going to go being pinched in place? Obviously if they're already broken when you remove them, you'll need to replace them. Your call.
ALWAYS set your pinion depth correctly first. THEN check/set backlash. Make those adjustments. If you're lucky, you'll get the pinion depth set up quick. Also, if you use new bearings, your inch-pound preload specs go up by about 10 in/lb over used bearings. Running preload should be somewhere around 20-25 in/lb with new bearings according to the CSM, (10-15 inch/lbs with used bearings). This varies a little depending on who you ask though. I personally would use a high quality bearing set (for example, Timken) to rebuild the rear end. Chances are Timkens were what it was assembled with.
Option- buy an extra NEW same-brand-as-you-plan-to-use front pinion bearing (nut side) and using a die grinder (takes a long time) or take it to a machine shop and expand the inner race so where it just slips on and off the pinion shaft. If you have to take the pinion out a few times, you'll be glad you did this part because you won't have to hammer it out. Plus, you can save the modified bearing to use in case you ever do another set with that same size/brand of bearings. Or sell/give the bearing to someone else who may need one.
You don't always have to buy every expensive tool people want to sell you to do a rear end setup. As mentioned, a cheap, but reliable, dial indicator with magnetic base should do what you need it to do and a few other "don't have to spend a boatload of $$" tools will make the job easier. You do need a good inch-pound wrench for the pinion pre-load measurements, but a good deflecting beam style will work. They're not as fancy as the Snap-Ons, but they will work. Your car won't care how much you spend.
One tool you may need is an electric impact wrench. They produce gobs of torque, and that front yoke nut is a bear to get off, and then sneak up on your final pre-load setting. To crush that sleeve you'll need lots of torque, but don't over torque it or you'll be putting in a new sleeve.
Always read up on as much information as you can from different sources if you've never tackled a certain maintenance procedure. ASK if you don't know or aren't sure. Rear ends aren't that scary once you know how it's supposed to go back together.
Here's a couple of ideas to read...
Jim Mitchke's another old-timer rear end guru that has grunches of real-world experience with setups. He runs a rear-end discussion forum at www.JDrace.com You may see him on other car sites as the user ID of "Monzaz".
There's a lot of good info there that may help. Again, it's the interwebz. Believe half of what you see, and be skeptical of everything you read until you're satisfied and comfortable believing. Sometimes Jim's advice contradicts the CSM, sometimes it doesn't, but he's actually been there and done it 1000+ times.
Lastly, and this is important. If after all this, you do NOT feel comfortable you can do this job, farm it out to someone you trust that is capable of doing the job. It's not something a beginner is likely to jump into and get it right the first time, but if you take your time, have some basic mechanical skills, and know how to use and obtain basic torque wrench, micrometer, feeler gages and depth gage tools, you can do it. Most importantly, do NOT rush.
