The 10 or 12 bolt designation refers to the number of bolts on the differntial cover. Count yours, and you'll know what you have. The 12 bolt is generally stronger.

The 9" is a Ford designation that refers to the diameter of the ring gear. 9" Fords, properly set up, are probably the most popular axle in hotrodding.

You don't need a posi axle - you need a rear axle assembly with a posi differential.

You have a couple of choices. If your rear axle is a 12-bolt, and it's in good shape, you could have a shop install a posi unit. If it's not in good shape, you'd be better off replacing the whole rear end.

You can get a 9" Ford or a 12 bolt Chevy, or a Dana 60 from Currie Enterprises. They can set any of them up for your car so they're a bolt in. They're not cheap, but neither is the replacement of a couple of rear ends after your 396 eats them. Do it right, or do it two or three times. . . .

http://www.currieenterprises.com/ces...t_rearend.aspx