As others have said, the bearings will be nipped in by the rear cap.
If doing (as you are) and in-car swap of the covers, I'd suggest shimming up each side separately. Any pre-load that's needed will be there already, so don't add any more. I find shimming in car like this easier and faster to do by trial and error than by measuring. You'll only what sufficient to ensure the side covers fit up flat with a gasket and no more.
I think the shimming is more about preventing torque steer than overloading the bearings. I agree these bearings are huge and probably hard to destroy.
Shimming the diff has zero to do with Torque Steer. The diff is mounted between the two diff bearings and the shims fit up against the mounting race of the bearings, therefore an shim added to either side will have an equal effect to both bearings. In any case, the output shafts (that connect to the wheels via drive shafts etc) are not directly connected to the diff hemisphere or it's bearings, by via a planet / output gear arrangement that are totally independent of the hemisphere's bearings, so what ever is being done to the diff bearings has no effect what so ever on the output shafts.
The need to pre-load these bearings may not appear obvious. It's twofold;-
i) To ensure that the bearings are loaded all the way around the races of them and not spot loaded.
ii) To remove any movement ('shake') from the bearings and hold the helix angle between the Pinion and the Crownwheel in correct alignment, otherwise the diff would tend to skew on power and the alternate way on over-run. This will result in these gears meshing only on the very ends (tips) of their teeth and not across the whole tooth face, resulting in very short life of these gears.
I agree that in isolation, these bearings are large, however, it appears they are this size, not because a big bearing is needed for any load they have, but to ensure enough surface area in the gearbox case and diff cap, the alloys that these are made from are soft and it wouldn't take much to have a smaller bearing bury itself in the alloy of the casing. This is noticeable on the diffs that were fitted up with the thin bearings that have around 2/3's the contact face.