GM rearends use the same number of bolts on the back cover as on the ring gear. Just count the bolts on the back, and that's what it is. (it's much harder to tell the difference between an 8.2 and 8.5" 10 bolts, so be careful here.)
All Chevy rearends use the same number of bolts in the cover as in the ring gear. All GM rears do not! There is an Olds rear that may have 12 bolts on the cover, but it's a 10 bolt inside, so be careful.
In regards to your post about faking a 12 bolt. True enthusiasts will always be able to recognize the 8.5" 10 bolt even if you change the physical appearance of the cover there are two distinct "humps" on either side of the housing (lower exremities) as you view it from the rear that would identify the rearend as a 10 bolt. Go proud with your 10 bolt or go deep ($$$$$$$) for another 12 bolt. Here is an idea for you. Buy a 12 bolt out of a later Chevelle and hack off all the odd shock and control arm mounting points and then weld on perches for the leaf springs. The perches from Moroso are only $20 and will allow the use of multi leaf or single leaf springs. You can get the Chevelle rear ends at most salvage yards for $150 complete from brake drum to brake drum.. I did this setup for a friend with a '68 and you unless we told you, you would never know the difference by looking at it.. The rearend is a little wider (1.5-2") but my friend runs the 15 x 8 ralleys and there is no clearance problem at all. e mail me if you want to know more. I can also tell you where to get rear discs for next to nothing..
SS, Whats wrong with your 12 bolt. Did parts actually break or did the posi go bad. You can buy a Chevelle 12 bolt cheap as Go69 says. If yours doesn't have broken parts but is only worn you can use the carrier from the Chevelle rear in yours and sell the Chevelle housing and axles to a Chevelle owner as spares. If not use the Nova 10 bolt. Theres no shame in having a 10 bolt. If you like to say you have a 12 bolt rear then its going to cost something to say it. If you don't need the burnt 12 bolt then put the 10 bolt in, be happy, sell the 12 bolt and buy something else your car may need. BTW, Scooter is right on with his info. Not many people know that some Olds 12 bolts have 10 bolt carriers