I prefer Fel-Pro gaskets and Permatex non-hardening gasket cement.
Use petroleum based gear lube and 4 oz. GM Position additive part no 88900330. I prefer Valvoline 80w-90 Part no VV831. In my experience, synthetic causes posi clutch plate chatter.

There is no drain plug. Big pan under the differential while loosening the cover. There is a fill port with an NPT pipe plug. Refill differential to the bottom of this port. Fill with posi additive first then top off with gear lube. Will take about 1 1/2 qts. Of gear lube.