I found a posi. diff

I have looked for a posi. I could afford for a long time. the owner said it was out of a buick but its a 12 bolt and big bolt pattern on the axel.isn't that a chevy? it doses look like it'll fit.will it??? I want it in my 71 gs. looking for advice,anyone.thanks.
If it is a 12 bolt it probably is a chevy. Some Canadian made Buick GS's had 12 bolts, but not many available. You would need to measure the spring perches and overall width.

It would be less expensive and a better rear to put a posi carrier in your 8.5" 10 bolt. They are very strong, and good parts availability. It already has bolt in axles too.