Help with posi for a Firebird?

I have a 1976 Firebird Formula, and it has an open diff. It`s not ideal for my dreams of hotrodding. I`m not exactly literate in rear ends, but I know I want a posi and I`d like it to be Strange. Could someone point me in the right direction of where to look for such a rear end? If it matters, the engine is the 400 and the trans is a TH 350.