Your '79 Trans Am has the corporate 8.5 10 bolt rear end. This is a very good rear end, but you have a 2 series differential. You will need a 3 series differential for what you want to do. You can use a ring gear spacer, but I highly recommend that you don't. Do it right the first time if there is any way that you can. Ring gear spacers should only be used if there is no other way to do it.
The Yukon Dura Grip is a good limited slip (posi) differential at a lower price than the Eaton Posi. I also recommend the Yukon installation kit with Timken bearings. If you can afford the Eaton Posi then I recommend Tom's Differentials for the posi, US Gear ring & pinion sets and installation kit. Tom has very good prices on high quality parts.