Glad this got bumped, because I need it. The guy that built my Duster was a pretty good wrench...but a Chevy guy and totally unfamiliar with Mopars. As such, he made some...interesting...part selections. I'm 99% sure I have a JS front swaybar, with probably-original .85" bars on the otherwise-totally-rebuilt (PST) front end. (Car is a factory 318.) He coupled this with Super Stock rear springs, too-short underride traction bars, and HD shocks...with the result the front is way soft, and the rear is like a hardtail Harley.
Obviously, most of this has to be dealt with.
Though a big-bumper car, my Duster is relatively light (~3300), with a 4-speed and no power options. I was thinking .99" bars (184lb), but this thread has me thinking I might want the 1.04" or even 1.09" MP bars.
Also, I dug up the old tech article about wheel rates from Mopar Action. For 35.8" A-body bars, the rates are...
.81 (MP drag)-82
.83 (slant)-92
.85 (315)-101 (No wonder mine handles like a marshmallow.)
.87 (340)-109
.89 (B/RB)-120
.92 (MP HD)-137
.94 (76 police)-149
.99 (MP)-184
1.04 (MP "HD solo")-224
1.09 (MP circle track)-270
1.14 (MP circle track)-323(!)
Feeling like a marshmallow can have as much or more to do with the shocks than the springs.
The SS springs - and the raised raised rear can make the back end feel stiff and jumpy.
The combination of softer front and SS springs when done right can provide excellent weight transfer to the rear when using race tires.
On the other hand as you observe, its a poor combination for turns and lane changes on the street and track.
The front to rear roll rates effect on handling relate most directly to front rear weight balance.
How high the spring rate ought to be will depend on the useage. Generally speaking, the worse the traction and rougher the surfaces, the softer rates will be more effective in maintaining the obtainable contact and traction. Tire choice, surface conditions, and use should drive the other decisions. The stickier the tire, the more rate will be wanted - not only for geometry and physics reasons, but some (many?) drivers are not so comfortable with high body roll.
Basically what I'm saying is what you do with the car's suspension should depend on use and purpose of the car. If you're running the drag strip and taking family trips, that's going to be different goals and compromises than just street use, etc etc.
Step by step, I'd look at the shocks to see what they are and condition. Likely change them out or adjust if they are adjustable.
Ditch the traction bars and check the shackle angles, the pinion angle, and drive shaft angles. For the rear springs to work, there need angles need to be in the correct range and helping the anti-squat. A pinion snubber against the floor will be a big help with drag race slicks, but otherwise shouldn't be needed.
For other than drag strip, bring the t-bars up to the 1" range for all season street tires. .99 to 1.04 is all similar enough I wouldn't sweat it.
For stickier tires and better roads, larger diameter is fine. I've moved up to larger diameters because I got stickier tires for autocrossing and still use it on the street with street tires, even pretty poor roads, without handling issues. The notchbacks tend to be a little nose heavier than fastbacks like a duster, so YMMV.
Note: T-bars in the 1" diameter range ought to have clocked hexes. Firm feel has that figured out when they started making them in house. I beleive PST does now as well (based on posts on this forum).
