Nope. Duster's and the hardtop or post valiant's do not have the same wheel well size. Your '68 has the same size wheel tubs as a '68 Dart. You won't fit a 275 under there without a 1/2" spring offset. The early valiants do have a higher wheel opening on the quarter, so, you may get away with a little more tire than an equivalent Dart, but it's not much. 245's are about as wide as you can go on most of those cars. On some you can get a 255 but the backspace will have to be dead on.
Dusters/Demons/Dart Sports and Barracuda's do have more room in the rear wheel tubs than the Dart's and Valiants. The Duster/Demon/Dart Sport body style, as well as the Barracuda's, can fit 275's with the stock spring locations.
