What I noticed on my '72 Duster was that the 3/4" rear offset shackles helped with the rear spring section, but not at the front.
Obvious if you think about it, but I didn't do that work.
Needed front mount to be inboard, but I'm not sure what's available there.
275/60R15s on 8.5" rims (4.5" BS) were still tight at the front section.
Oddly, my '74 with 8" factory Rallyes & 275s cleared the springs fine at both sections with a .250" spacer. Never rubbed. Moved spring eye back .250".
My '70 is like Cab's deal; springs moved to the rails & mini-tubbed & stretched a bit (just cut), so a 13x31 slick fits fine on a 15x12 rim.