It is hard to find a rear in decent shape for a 35+/- year old car. You can search around, or check realoldspower member 507olds who specializes in GM diffs (Brian Trick), or go with a new reproduction strong replacement rear. The last will run you about $2000 when you are done, most likely. You can check
www.quickperformance.com for 9" ford bolt in rears and also a vendor of Moser who makes repro 12 bolts. A lot depends on what you plan to do. what ratio, etc. A stick car with slicks needs a lot stronger setup than an auto with street tires, etc.