When should you upgrade torsion bars?

Mopars are notoriously undersprung. Even in an era where "good" handling cars scraped the chrome off their door handles going around a corner, Mopars were considered inadequate. Beefier torsion bars help solve this.

When you add stiffer springs to the rear, you make the handling worse, because more of the "roll" experienced in cornering occurs at the front, further limiting the tire's ability to get you around the corner.

So unless your Dart is going to be a drag-only car, you will have a much better driving experience with bigger torsion bars.