are you doing the narrowing yourself. If you are you will need the correct tools to do it one would be a lathe to trim taper and respline the axles after the housing has been narrowed.
You will also need to build a jig to set the axle in to keep it strait in all directions.
The easiest way to find your width is figure how big of tires and where they fit in the wheel wells (if you have the wheels that would be even better) and counting the backspacing and such measure where your axle would mount to wheels and build your jig to that width
you can move your springs in to allow for the large meats and ive heard that you would want to angle your shocks with the bottoms out to improve stability.
well thats all of my knowledge have fun