I need to know the backspacing, but you can also figure this out easily enough.
Basically take a back wheel off, lay it face down. Measure from the back mount of the wheel to the edge of the wheel (straight up in the air) the distance from the back edge of the wheel to the wheel mount is the back spacing. The rim width minus the back spacing is the front spacing of the rim. The front spacing + back spacing gets you back to the full rim width.
The front spacing needs to be 3.5 - 4.0 inches (mine are 4.0 inches if you want to look at pics via my signature (high res pics).
Read this article to see more about backspacing, etc etc and what it is (with diagrams)
Mustang Wheels and Tires FAQ