As far as what width you need is concerned, how does your wheel/tire combo fit now? Most will sit a bit towards the outside.
Since you're buying a new rear this is your chance to move the rear wheels/tires in a tad to better center the wheel from side to side in the wheelwell. This will allow you to use the maximum width tire on your rear wheel.
Just measure carefully, and remember each side of the car will be slightly different. The body can sit 1/2" to 3/4" different from one side to another due to production tolerances.
Scroll to the bottom of this page, there you'll find a drawing of a typical rear wheelwell. Instead of using a custom-made wheel to center the rim you'll be narrowing the rear to do the same thing.
However if you're happy with the way your current wheel/tire combo fits then just order the stock width rear. This way if you ever put the stock rims (Rally I wheels) back on the car it will still look 'right'. Decisions, decisions.
