The diffs can be specified with your choice of lockup and intial torque, though most go for the default setting by OSG. It's really only suggested to deviate from that if you have substantial track time, notes, and datalogs to back up what changes you're looking to make.
For the sidelash shims, in some rare instances a different size shim may be required from what is provided from the factory. This is not something you can predict at the outset, you're installer will only know it when he goes to install everything. Shims are offered in a range of sizes and available from the dealership directly. In 99% of the cases I've been involved with (more of these than probably anyone else in North America for the Z/G platform) no other shims were needed as the factory ones were able to be reused.
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