Why? For resto purposes? Or just cause you want a spare?
When I was looking about 2 years ago (mine had a stock 6 cyl, 7.5" 2.47 geared posi in it) every one I found had a lot of miles and needed to be rebuilt, and prices were in the $2500.00 and up range. Parts alone would have been another $500.00 or more, plus labor. So I called Quik Performance and ordered a 9" bolt-in housing, big drum brakes (better for holding boost on the line) and axles.
I found a punkin for $50.00, bought 3.50 gears (half the price of 3.42s), a new posi, and bearing kit. Total price was only around $1600-1700 because I did the labor myself and shopped sales and specials. That's about half of what it's going to cost to do an 8.5.
The only fly in the ointment is that although Quik Performance has been aware of it, the angle of the lower shock mounts is waaaay wrong and puts the shocks in a serious bind. I used thick wall tubing and the originally supplied shock studs to correct this. It does require welding the way I did it, but I made up a couple of extra sets of lower mounts in case anyone needs them.