Eaton makes a good posi unit. Use a 3-series posi carrier with the 3.73 gears. If you plan on driving it on the street, get Moser's C-clip-type axles. C-clip eliminators are leaky, and the Moser guy I talked to said they won't break nearly as easily as stocker. A rear end girdle such as TA Performance's TA-1807 would also help keep it together.
Weld the axle tubes to the center section, all the way around. They're just pressed in, and they'll slip around and cause leaks and all kinds of squirrelly stuff if you hit them with any kind of sudden load. Be sure anything welded to the axle tubes by the factory is FULLY welded and not just tacked here and there.
Operator education: avoid neutral drops or clutch side-steppage.