The slant 6 bars are so undersprung that the weight of the car results in a significant about of twist in the bar. Over time the /6 bars can become bowed as a result. It's not unusual for the /6 bars to be bowed like that if they're originals. Buy new bars if you want to run that little wheel rate, they have a lot more movement and twisting than bars with a decent wheel rate. Running 40+ year old springs that see that much twist and movement is asking for trouble eventually.
And torsion bars are not just bar stock. They're spring steel and heat treated. Why you'd rather spend $5k on a coilover set up instead of just spending a few hundred bucks on good quality new bars is beyond me.
