The first thing that you need when narrowing a rear end is the car (or truck). The next thing that you need are the wheels and tires. You need to put the wheels and tires under the car and get them into the correct position and measure between the wheel mounting surfaces to get the total width of the rear end. Subtract the thickness of the brake rotors or drums. This is the easiest and most comon way to do this. You can start with the rear end, but you must make some carefull measurements to get the wheels and tires right.
If you want to use these used rear end parts then you will have to get another set of wheels and tires to fit. You might be able to send the axles back to the manufacturer and have them cut down some more and resplined, then narrow the housing again. Be sure to get the pinion offset correct for the car that it will be going in.