Front's should be good, in fact, you should be able to run the 18x9's and 255/40/18's up front with 6.18" of backspace if you want to have the same size tires all the way around. You will probably have to have the hub register of those rims opened up, or the hub on the stock rotor turned down to get them to clear though.
In the back, I would think you'd need less backspace except that Peter (GmachineDartGT) is running 18x9's with 6" of backspace and 255/40/18's with a 3/4" spring offset out back. Considering that the 3/4" MP offset and Dr. Diff's 1/2" offset both move the springs in about a 1/2", you should clear if Peter's car is any indication. I'm not sure he runs rear disks either, Dr Diff's kit adds about an 1/8" to the width, so that would actually give you a little more clearance to the springs (and still leave plenty to the quarter).
http://www.forabodiesonly.com/mopar/showpost.php?p=1969902119&postcount=72
