In the back it's because you've got a Dart. The Dusters/Demons/Dart Sports have a full 1" extra space going to the quarters. Even with the 1/2" spring offsets 275's can be really tight on the Darts.
In the front they should fit fine, I had very little issue with the 275/35/18's beyond the front lower corner of the fender. The fender roll was only because of how much I've lowered my car, and like Craig has shown the only rubbing I really ever got was going in and out of parking lots- bascially the combination of having the wheels turned and suspension compression at the same time. Since I've rolled and pushed the fenders they're well clear. I've considered going to 285/35/18's, but I'd need new rims up front since the 18x9's are too narrow for that.
