The spring rate of the bar does not significantly change or get "softer", but will deform after many cycles. To compensate the factory put an adjuster on one end. Eventually the spring will fatigue and break, but at the point of failure the spring material will be harder (stiffer) than the rest of the spring.
As others have said, these cars were, by current standards, quite undersprung from the factory. They really were that soft. Add worn shocks and suspension components and sloppy handling is the result. What condition is the rest of your front end in? Even a basic front end rebuild will produce a "night and day" difference. Since your front end is "bouncy" it shows that the torsion bars are working just fine (it also shows that your shocks are shot).
Assuming you aren't made of money, I would suggest you do the following: First, rebuild the front end with quality parts and good shocks (rear spring bushings and shocks too). Next step would be to add a front sway bar (huge difference). Then consider replacing the torsion bars, .870-.920 fine for cruising, something more like 1.00 for performance handling. Along the way you'll want to consider front disk brakes (I'd do this with the front end rebuild or soon thereafter), and a wheel/tire upgrade. Be honest about your budget and what you plan to use the car for.