On the rear body of the S197, there is a bracket spot welded to the inner fender well that provides support for the bumpstop under hard compression. If you fit 18x11s onto the rear of the car and are experiencing clearance issues, this is the first spot to check.
1. Leave it as-is. If the rubbing only occurs with the car up in the air, you likely won't experience this situation while driving (unless you become airborne) and there will be no negative effects.
2. Remove or relocate the bracket by drilling out the spot welds, sanding them down and painting over the effected area. A spot weld drill bit can be dangerous so this should be done by someone with experience. If you rely on full functionality from your bump stops on the axle, they will need to be relocated inwards using a kit found here: http://ccmustangbrackets.com/product/s197-bumpstop-relocation-brackets/
|OEM Bracket Removed
3. Use rear spacers. However, this could cause the tires to sit past the outer fender lip and rub with certain lowered suspensions. This should be the last route considered.