Bill gives some sound advice, you will have to carefully measure and 'play engineer' to really find out what you need. I would also add that mounting a bare stock rim on the rear and loading up the trunk with weight will help you see visually how much wider you can go.
The stock rim will help give you a reference point for width and backspacing. Loading the trunk almost to the point of bottoming the suspension will show you what the clearances would be on a bump you might (will) encounter.
In this thread I gave some help to another board member on a similar subject, you might want to read through it for some more insight:
http://forums.performanceyears.com/g...1&m=2446059391
Good luck.
