Is this a symptom of worn out torsion bars?

If you are running base slant six bars with a V-8, thats your problem. Even with a six, they are way too soft.

Any spring will eventually fail and break, but the rate will remain constant up to the point of failure (actually it will increase dramatically at the time of failure). A leaf spring will sag or take a "set", but the rate will not change appreciably. The spring has simply become bent over time. The same spring can be re-arched, or bent back to it's original camber, and it will be the same as new. In some cases the camber can be increased beyond the original amount, but external suspension geometry constraints will limit this. Shackle angle in particular.

Think of a torsion bar as a coil spring that has been uncoiled. It too will take a "set" in time and will become permanently twisted. This is why the thoughtful engineers at Chrysler put an adjusting screw at one end. Even so, they don not lose their "spring" over time. If you have added a V-8, you are asking the spring to carry more weight than it was originally designed for. With that added burden, the spring will deflect more on each bump and you'll be whacking the bump stops harder and more often. If Mopar had felt the slant six torsion bars were adequate with a V-8, they wouldn't have specified larger, stiffer bars for V-8 cars. Like most cars of the period, the front suspensions of our A-Bodies were really softly sprung, and benefit from much firmer spring rates. The "cloud soft" ride went out of favor because it caused mushy handling and frankly was unsafe.