Chrysler had several size torsion bars and different leaf springs for every model.
Which ones got used on your particular car depends on the engine it came with, and how it was optioned. There were options for heavy duty and fleet use.
To reduce roll without increasing spring rates, use 'anti-sway' bars.
Again, your car may already have them.
Adding stiffness to the rear springs is not a good idea unless it is specifically to carry extra weight.
Everything else being equal, increasing rear stiffness makes it more likely the car will fishtail and even spinout on slick surfaces. Don't ask how I first learned this...