You are not looking in the right places. Eaton makes their Positraction diferentials for the 8.5" inch rear is offered in the stock 28 spline and in the 12 bolt sized 30 spline axels. there are others (such as Moser or Strange engieering) that can modify the Eaton to use a 31 spline axel out of a nine inch (which is good because you can not race with a C-clip holding your axels in place regardless of the size or spline count).
Go with the 8.5 rear from the 91-up B body. Big drums, or discs with the 94-96 Impala SS or Caprice police package (option 9C1). Stock 30 spline, lots of ratios available, and proven to hold your power in a 4400lb B body at the strip. Less HP to turn than a 9", too!
1971-'76 uses parallel four link arms with a Panhard bar, the 1977-'98 models used an offset four link without a Panhard bars as the uper two arms that attach to the cast iron housing like a Chevelle rear end are angled in towards the cars centerline. It is about the same size (width) as the earlier rears.
Once you have a 30 spline carrier with your gears installed you can slip in factory forged steel axels or the aftermarket billet axel that is machined out of a different alloy for 20% more strength. Both are the same length.