You generally take the rear end out of the car and whack off the ends of the axel tubes to cut down the overall length of the assembly (to center the wheel in the wheel well). This assumes your tire will fit inside the wheel well without rubbing, if it rubs then you have to tub the wheel wells as well (add more sheet metal to make them bigger), while you are shortening the axel. Of course just whacking off the ends won't work; so you cut out what you think you do not need and then reweld on the axel tube ends (the parts the brakes bolt too that you whacked off) and order new shortened axels to fit the shorter housing. It all has to be welded in a fixture or jig to keep everything straight and aligned.
Finally when you have the rear the length you want you try and bolt up your narrowed rear end and if it fits, hooray your done! Other wise you have to cut off the back half of the car and rebuild the frame and floor section to accommodate the narrowed rear end in terms of control linkage (most folks who have gotten this far go for a four bar using chromemoly tubing and heim joints). After you reweld the frame rails closer together and get the suspension linkage to work (properly adjusted to be square and parallel to the front axel) you have to stiffen it up with new cross-members and then install new sheet metal for the floor. Don't forget to make a new narrower gas tank to fit between the new frame rails.
A lot of work and expense for a few fat tires Huh?