If you've already got a "limited slip" rear, you've got a "posi"..... Positraction is just GM's name for a limited slip rear. You could pick up a set of gears and an install kit for $300-350. Cost another couple hundred for a good install.
As far as the cost of a new Eaton, try the vendors like Summit or JEG's, or anyone who advertises in the popular Chevy mags... DTS, Randy's, etc.
This belongs in "Classic Tech" ......
Moving it there
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Fred
94 Formula A3: 381/TH400/N2O
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11.513@115.59 on motor; 11.162@127.67, 1.643 60' on a 125-shot. Going with a 275-shot this year