Get one of the 8.5 Olds/Buick 10-bolts, nearly as strong as a 12-bolt and they come with some better features. They bolt right in, at the most you'll need to shorten the driveshaft a tad and change the rear U-joint. It will be same width as your '68 BOP rear.
The stock 28-spline axles are true heavy-duty units and the stock posi is fine as long as you're not drag racing with slicks. The axles are also a tapered-bearing design that bolt-in,
no C-clips like a 12-bolt has so no C-clip eliminator needed.
I have one in my Tempest with 3.42 gears, but I got a little spendy and upgraded to an Eaton posi and Moser 30-spline axles.
Here's nice one for sale, from the same fellow that did mine for me:
http://forums.maxperformanceinc.com/...d.php?t=617520