Good luck with that! Mine's already been done - a ring gear bolt ended up backing out and banged around inside there for a few minutes and then got shot out the back. I think I had the half shaft yokes replaced at the same time. Once you get that guy rebuilt, you will want to change the oil on a regular basis. The only way to get the old lube out is to pump it out with one of those pumps you get an auto parts store. Mine has never worked very well - very hard to get it primed and it's always such a mess. Getting diff lube out is the ONLY thing I will use it for now.
Don't forget to use the real GM posi additive when you do change that oil. You can get it at any Chevy dealership. I use two bottles in mine.
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1976 Silver/Firethorn. L48, 4spd. Original 2 bolt, vortec heads, 9.4:1 CR, Speed Pro Cam: 224/224@0.050, 112 LSA, Eagle Steel Crank.