Need some help controlling body roll

Professor Fate said
Okay, I'm going to muddy the waters a bit here, but try to follow my reasoning...
If you truly do have 4.5" of backspacing on those 15x8s, those 255s should fit, no problem, PROVIDED YOU HAVE A STOCK WIDTH A BODY REAR END! From the look of your pictures, the amount of axle outboard of your springs makes it look to me like you've got a B or E body rear end in there. You need to break out the tape measure and figure out what you actually have- the spring perches were probably moved for installation in your Duster, but you need to verify the rest of the measurements. Use this thread: An accurate 8 3/4" rear axle width list
Once you have confirmed what you have, then and only then will you be able to figure out what offset wheel you will need to center your tire in the wheel tub. Until you know what you have, anything else is pure speculation.
As for your rear springs, they're toast. If you've got shackles that long on it and you're still having issues, they're done. Four leaf is a standard-duty spring, and they're 50 years old now. Lots of threads on this site about good vendors for replacement springs.
But first, confirm what rear end you've got in there.

Yup, I totally agree. If that was an A-body 8 3/4 with 15x8’s that have 4.5” of backspace then 255’s should fit just fine. That’s not an A-body 8 3/4. I would bet that’s a B body rear axle in there.

That brings up another issue though. The rims may not have enough backspace for your rear axle, but they are the right rims for the car. And to get more backspace on a set of 15x8’s you will likely need custom backspace wheels.

The shackles have to go, those extended shackles screw up the suspension geometry and don’t work all that well anyway. Fun fact about the leaf springs though, they were designed to be zero arch springs. So the fact that they’re almost flat at ride height means they’re not far from where they normally are anyway. Guys that run SS springs are used to seeing a heavy arch on the springs, but that’s not stock. The previous owners no doubt tried to raise the car to make up for the fact that the wheels stick out too far and hit the body. With the right wheels, you wouldn’t need to raise the car.