You will not find one in a salvage yard. The Chevy 12 bolt rear ends are very hard to find now, and I know that there are people who are being paid to search the scrap metal piles for any Chevy car 12 bolt rear ends that they can find. There is a guy here in KY that has several people in many different states that he is paying to search for them.
The Chevy car 12 bolt was only produced from '65 to '72. It was used in the full size Chevy from '65 to '70, the Camaro from '67 to '70, the Nova from '65 to '70, the Chevelle and El Camino from '65 to '72 and the Monte Carlo from '70 to '72. There were a few GTOs that got the Chevy 12 bolt, but not many.
The truck 12 bolt is similar, but the ring gear flange on the differential is offset different than the car 12 bolt. The Eaton Posi that was used in the truck 12 bolt was weaker than the car Eaton Posi.
Why not just get a new Eaton Posi and have it done? There is also the Yukon Dura Grip, which is an almost exact copy of the Eaton Pois for a little less money. If you find a used one you will probably have to rebuild it anyway.