What gear ratio is in it? If it has a 2.73 or 2.56 then you afre going to have to find a used 2 series Eaton Posi. If it has anything between 3.07 and 3.73 then you can use a used 3 series Eaton or get a new 3 series Eaton Posi. If it has the thin 4.11 to 6.00 gears then you are going to need a 4 series Eaton Posi, new or used.
All of the Chevy car 12 bolt rear ends use the same internal parts, Chevelle, Camaro, Nova, Impala, but finding a used one in a salvage yard will be almost impossible. The 12 bolt was only made from '65 to '72, and just about every one has been snatched up. If you can use the 3 or 4 series then I reecommend just getting a new one. Tom's Differentials has good prices on the Eaton Posi and he can get you the shims and bearings that you need to install it.
If you are wanting to change the complete rear end then the '68 to '72 Chevelle, El Camino and '70 to '72 Monte Carlo are the only direct fit 12 bolt rear ends. The '65 to '67 Chevelle and El Camino 12 bolt rear ends are 1 inch narrower, but still bolt in. They will also bring a higher price.