Practically speaking there really is no need for a torque spec for the diff cover bolts. These are bolts with an 8mm shank - tightening them with a wrench or ratchet of about 8-9 inches in length means using your wrist and forearm --probably something like 10-12 pounds. No need to put "body" torque on to those screws. Tighten them going in a crisscross pattern -- first snug and then with a final tightening.
Any good parts store--indy or chain should have individual copper or aluminum gaskets in an assortment box under the counter. -- Dave