i think most of the 6's came with an open 3.23. some of the 4 speed 6's had a 3.55.

you can just change all the guts from what i've heard.

there are three different carriers. 3.36 and up, something ranging from 3.23 to somewhere slightly below 3.00, and another below that.

auburn and eaton both make BOP posi carriers. the eaton is s'posed to be better, and cost slightly more. they are both on ebay right now.

cost depends on how many parts back there you want/need to change. you're looking at anywhere from $350 to over $700 acording to ebay, and then there's installation (prob'ly another couple hundred unless you do it yourself). make sure your rear is still a BOP unit. the chevy 10 bolt is different, and came in 8.5 and 8.2.