There are three easily available kits to do this. 1) Mopars 3/4" relocation kit. Bolts in and retains stock alignment, it simply moves everything over 3/4". 2) Dr Diff 1" relocation kit. It also bolts in. I have not seen these in person, but I think it moves the springs geometry from splayed to parallel. 3) Mopar 2" relcoation kit which requires cutting the front frame rails and welding in new mounts. This kit moves the springs in far enough that you can min-tub the wheel wells. Any of these kits will create enough room for a 10" wheel easily.
Relocating the springs inward does change the leverage ratio on the springs, so the further in you move the springs, the higher the spring rate will need to be to compensate for the additional leverage. However, with that said, it isn't a huge change and most drivers would be hard pressed to be able to tell the difference between the two. I did the calculations for a member on here once, and while I don't remember the actual difference, it wasn't huge.
Also, keep in mind that the factory set up of the leafs in an E body is a splayed design. That is, the front mount is narrower in width than the rear mount. This design increases the roll resistence already inherent in a leaf spring, which is a good thing. Moving the springs to a parallel position does reduce some of this roll resistence, but like above, it is a slight change that most people probably won't notice. However, combine several of these factors together, and things may feel a little softer in the turns. You could overcome this with the of a sway bar. If you do move the springs in, you may be able to use a B body rear sway bar instead of an E body bar because they are somewhat narrower in width. But, it depends on the make and mounting method on how well the bar will fit.
