Rear Sway Bar?

I agree with autoxcuda what you're describing sounds more like you still need to stiffen the front end more, not the rear. But that's just based on your description.

Whether or not you'll need a rear bar depends entirely on the rest of your suspension set up. What torsion bars are you running up front? Which aftermarket sway bar did you install up front? What springs do you have in the back? And what tires are you running?

Typically, because these cars are usually heavy in the front and light in the rear they need a lot more spring rate up front. The factory set up was very undersprung in the front and only slightly undersprung in the back, which is why on a lot of A-bodies people have issues with oversteer if they mount a rear sway bar. But it depends entirely on how your car is set up, my Duster makes pretty good use of a rear sway bar but I have a much higher spring rate in the front than I do in the back, and run wider tires in the back than I do in the front as well.