Or you could do 1/4 of the work that Bob suggested: jack it up and look
for the stamped in code on the bottom of the differential flange.
According to Chevylay your '77 ought to have one of the following:
OA - 3.08
LR-OD - 3.36
OB - 3.55
OC - 3.70
Here's waving to ya - \||||
'67BB & '72BB
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