Need some help controlling body roll

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.