They don't move them that far, only about a 1/2". But that's all it takes. The offset hanger/shackle kit makes the wheel tub the narrowest part instead of the springs.
I have the offset kit on my Duster. With my 295's I only have about 3/8" to the springs, but I've never hit them. I have, on the other hand, had the tires rub on the inside of the wheel tub before I installed my rear sway bar. If you keep the stock wheel tubs, the 1/2" offset is all you need. Even if you did do the full 3" spring relocation it wouldn't buy you any more space without a mini-tub.
