You are correct to be worried about the shims. In theory the Quaife machining dimensions on their part measures exactly what the stock unit was for the few dimensions I checked but that does not guarantee it will be a pure remove and replace. I do think you have a great chance of getting away with no change since you are just doing the Quaife. It couldn't be off by that much and you could buy a couple of shims ahead of time to cover the possibilities but they are not cheap (like $20 each!) and I picture you buying about 4 to cover looser or tighter backlash. If you hope for the best and wait to get them after you find you need them, get some advice from someone more experienced on this than me. But I would guess that you could put your ring gear on the Quaife with the old ring gear bolts and do a test fit. If it didn't come out right you could MAYBE put the gear back on the old diff with Loctite (probably red even though it will be hard to get back off) and drive it that week until the parts come in and you can work on it the next weekend. A little risky but may be OK? Some things just don't go ideally but will work anyway.
I would come up with a decision on that first. There is nothing worse than taking your car apart for something major with no backup plan.

You need to take the original rear end out and measure all of the specifications on it so that when you put the new pieces in, you know what everything should measure. You need a dial indicator with a magnetic base and a small range deflection beam type of torque wrench. There are a couple of decent ones on Amazon for about $25.
You will need something strong like a 3' long piece of angle iron to attach to the input flange so that you can get the holding power you need to remove the pinion nut with about a 40" breaker bar. You need to make a mark on the nut and shaft and count exactly how many revolutions it takes to get it off so that you know exatly how many to put it back on plus about 1/8-1/4 extra turn to get the preload about the same.
I was shocked by how much holding power it takes to hold the diff while you tighten the ring gear bolts to torque spec plus 30 degrees more. I made a holding jig with dowel pins after 3 failures holding it in a vice trying not to damage it.
Also I had trouble centering the gear on the Quaife because there was a little loose fit on mine I didn't like.
I could ramble on and will if you want the help. Gotta go for now...
Think about some of this and decide if you want to do it on your own.

We can give you plenty of support. Several of us have done this recently.

 

You need to order a new crush washer and pinion nut. Didn't see that above on your list.
You also need gear marking compound. You can get that from your local GM dealer.
A good way to drive in the seals you said you are buying is to go to a hardware store with a large PVC section. Take the seal with you and select a PVC adapter or anything in the PVC aisle that fits over the seal and lines up with the shoulder of the seal that you can hit with a rubber mallet on the other end to drive it in.
Don't follow the exact advice I said about counting the turns on the nut and putting that back on exact. I was confused about when I had to remove my front seal and replace it and did not want to use a new crush washer. I got some advice from a pro on cheating a little. You should use a new crush washer on yours. When I pictured you doing this with used bearings and not upgrading the gears I remembered the trick I told you before. Just do everything the manual says.
Are you replacing all of the bearings or only the ones on the Quaife? Do you have a hydraulic press, bearing splitter, and two jaw puller to remove your bearing to get the old crush washer off? And also to put the new bearings on the Quaife?
If you don't have the special tools you need this can turn out to be a job that costs the same or more than a pro to just do it for you. But you better find one that understands what he is doing. Most, unfortunately don't seem to remember how to do this. Try a local race shop vs. a regular garage to start out with. Let us know what you decide and what questions you have.

Have you read the RFD section in the FSM?

 

Thanks, bjr.
Damn, I just priced some tools I would have to get in order to do this right... It puts me around $400 (I don't even have a vise or a solid workbench to mount it to). Might have to talk this over with a guy I know who owns an import shop and used to race Datsuns. He said he's rebuilt many a rear end on Z cars. Really wanted to do this myself :/ But major factor was saving $500 that would go toward true coils and suspension arms. Oh, well.
I will update the thread.