The shims behind the brake backing plate are strictly for setting the end play for both axle bearings and won't change the ring and pinion settings.

The shims are typically found only behind the right backing plate, but there is nothing wrong with putting shims behind the left backing plate or behind both backing plates.

To install the locating key, install the drum on the axle while lining up the 2 keyways. Push the drum on until it fits tightly. You want the drum to fight tight but not so tight as to need the puller to remove it in case you need to reposition the drum if the keyways don't line up. Then install the key with the tapered side in 1st with the taper facing down at the axle. You should be able to get the tip started in the drum keyway and then tap the key almost all the way in with a punch and hammer. You can leave a little hanging out of the drum as the washer will push the key the rest of the way in when you tighten the axle nut.
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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com