No idea about the Charger. Maybe talk to some of the guys at the different Mopar parts houses. Mancini, Mazzolini, Firm Feel, CAP Auto, Hensley. They usually help if you ask nice. If they don't know ask them who does.
If you want to find out for yourself.
-Figure out how tall of a tire you want to run.
http://www.moparts.org/Tech/Archive/wheels/11.html
-Set the rear axle on jack stands so the suspension is at ride hight and pull the wheels.
-Tie a weight to the end of a string a few inches longer than your tire hieght.
-Mark the outer fender well half the tire hieght above the axle center line. You can use a coat hangar bent at 90 degrees to make it easier.
-Tape the string to the fender well where you marked. This marks the where the tire will sit at ride hieght.
-Measure from the string to the leaf springs to see how much width you have to work with. Subtract how much clearance you want between the outer fender and the tire, then do subtract how much clearance you want between the tire and the leaf springs. That will be your tire width.
-Now that you know how wide of a tire you can run, you can figure out how wide of a wheel you can run. Wheel width is measured between the lips so add an inch for total width.
-Subtract the wheel width from the tire width and divide by half. Add that to the clearance you want between the tire and the outer fender. This is how far your wheel will sit from the fender. Subtract that from the distance btween the brake hub (where the wheel will mate with the brake) to the string. Thats your backspace.
Make sure you factor in room for suspension travel.
Hope this helps.
