What torsion bars do you have up front? Is it a factory front seat bar or aftermarket? How are you using the car and what tires are you running?
I run front and rear sway bars on my Duster, my rear bar is actually a larger, 7/8” e-body Hellwig.
The important thing is that you’re adding the sway bar to balance the car. The whole deal about A-bodies not needing them is complete garbage, but it does depend on how the car is set up with the rest of the suspension and tires.
Factory cars are all set up to understeer. It’s easier to drive like that and requires less skill. Having a car that’s on the edge of oversteer is faster for handling, but requires more driver skill to manage. If your car understeers, a rear sway bar will improve that. If you’re already into oversteer, well, a rear bar will make that worse.