The above is correct; a normal leaf spring assmebly will twist a bit sideways under hard cornering, particularly at the very front and rear ends where it tapers down to one leaf. A thicker mono-leaf will resist this better as it does not taper down. Additionally, the shackles are another source of sideways motion. The panhard rod (track bar) is not essential for a leaf car as it is for a 4 link, as the springs will eventually tigthen up and stop the sideways motion. But for consistency and axle stability, it willl make a great improvement.
Consider this from Sam Posey who drove one of the Trans-Am Challengers fielded by Chrysler: " It was better, but stiffening the frame exacerbated the cars other weakness: the geometry of the rear suspension. As I braked for certain turns, the rear brakes locked up, causing the axle assembly to jump from side to side, leaving skid marks on the track. All I could do was to brake gently, which of course let anyone following me go right by." Straight from the horse's mouth.....
There is another critical fuction of the track bar if you are really engineering for handling. It allows you move the rear roll center up and down and thus change the roll axis of the car, which can be used to hlep tune understeer/oversteer. With the standard leaf spring setup, the roll center is cannot be moved without changing shackles or re-arching leafs, and is usually higher than desired. If you use rather loose spring bushings in the rear springs and hard bushing or heim joints in the panhard rod, the panhard rod takes over all of the side-to-side locating function and the roll center is determined by the panhard rod settings, not the leaf spring connections to the chassis.
It is pretty easy to build a plate for the axle connection end of the panhard rod; it can just replace the under-spring plate for the leaf spring on one side. The chassis end connection of the rod takes some fabrication and need to spread the load along thr frame rail a bit. If you can find Fred Puhns "How to Make Your Car Handle" book, look at the photos on page 153 of panhard rod mounts on a leaf-sprung car.