The Rapid Transit System advertisement (1970) lists a 3/4" rear sway bar as standard on the 'Cuda.
reprinted in North, Barracuda/Challenger A Source Book, Vol. 2

As a side note, Car and Driver's test car was an early AAR that came without rear sway bar but extra stiff rear leafs to try to make up for it. Handling on the track was a handful - a lot of work for the drivers. It would seem those cars really are best with the rear sway bar package.

With the A-bodys, road racers have often found a rear sway bar unneccessary to dangerous. On a street car, there will be less body roll - which most people equate to better handling. But on the street, the tires are rarely at the limit of maximum grip. However as speed increases, or traction conditions get worse (rain) the limits of tire grip are reached. Under these conditions, the stiffer rear roll rate makes it more likely the rear tire's grip will give up before the front tire's grip. Then the rear goes past the front as you watch!

So my advice is that no matter what the car, and use the best factory package as a starting point or goal. Then make any increases to the front roll stiffness (t-bars, anti-sway bar) before increasing the rear roll rates (leafs, rear sway bar).