Are /6 torsion bars the same as 318?

There are only 1 or 2 after-market brands. I recall one size is most popular and thus more available and cheapest (1" D?). Used factory parts can be much less. One guy at a local Mopar show had several sets on the table for $10/set, so I grabbed the thicker 4 bbl ones (0.87"D) for my 65 Dart 2 bbl (0.85"D), then moved those to my 64 slant (rebuilding both front ends anyway). But, some used ones are permanently sagged (twisted), and don't swap L & R to fix that. Early A's are lighter, which helps. In the same vein, if you lighten the front end your 0.85"D T-bars may become fine. One way is to ditch the beastly RV2 compressor for a Sanden. Others are aluminum radiator and condenser, aluminum heads or intake, ... Think long-term before changing. New heads is for V-8 guys, we would love an aluminum head for slants.

The factory spring rates were too soft and "boat-like" for present tastes. That was long before BMW's popularized handling for the general public. Current cars may have gone overboard with stiff suspension and rubber-band tires that give too authoritative a ride.