NO tubbing out!

Offset spring shackles will give 3/4 of an inch more room. While that will help a small amount to really gain much you need to relocate the springs to the frame rails to get 2-3" inches more clearance. With that you can stuff some pretty wide tires under it.

If you don't want to go through that much work careful measuring of the backspacing and tire wall width (which can vary from mfg. to mfg.) can net some fair size tires. I have 8" wheels with 3.75" backspacing and 275/60/15 tires under my Cuda which has about the same amount of room as a Duster and they fit with plenty of room. From my measurement it looks like 4.25" backspacing is all you can fit without relocating the springs either via the offset shackles or total spring relocation. With 4.25" backspacing a 8.5" wide wheel fits without sticking out too much. You could probably go 9" on the wheel width if you carefully choose tires with a narrow side wall.

Height is another issue to keep in mind. 28" tall is as tall as you can go without the tires hitting the front/lower section of the wheel well opening. The only thing you can do about that is modify the wheel well opening or space the rearend back a little but you have to keep in mind when you move the rearend back your pulling the driveshaft yoke out of the trans. so you might need a longer driveshaft to make up for it. You also have to watch the rear shackle angle to make sure it's still ok. If you only space it back 1/4" you'd probably be ok but that's all I'd chance it.