Id also suggest starting out with a call to DoctorDiff. He has new nodular iron cases and everything to go inside them:
http://www.doctordiff.com/8-3-4-489-case-third-member.html

His price is $1375 with everything new, Nodular case, Powr Lock, gears, all set up. The nodular case is important as it is 50% stiffer than a common cast iron case. Adding his partlist prices I come up with $1041. Throw in $175 for gear setup time and $200 profit and there's your price.

With something like that you get what you pay for.

In my local Craigslist I am seeing used 8 3/4s with "Posi" to be a little more than $500. That's all greasy, just pulled out of the axle and set in a can.

R.

In your shoes I'd be looking for a Dana 60.