An 18x10 can work, but you have to get the backspace right. With the stock spring locations, the absolute most backspace you can run is about 5.5". That's because with an A-body 8 3/4 and BBP axles and brakes you should have about 6" from the wheel mounting surface to the springs. Since the section width on a 275 is about 11" (same as the outside to outside measurement on an 18x10), you don't have much tire overhanging the outside lip of the rim so that can work. That would mean using an 18x10 +0 though.
Realistically, you'd be better off with an 18x9 or 18x9.5, as that will squeeze the tire a bit. A 275/35/18 is about the limit with the stock spring locations on Demon/Duster, although if the quarters have been rolled you may have some extra room compared to stock. For an 18x9 or 18x9.5 you'd be looking for a +0 to +6, which works out to a backspace between 5" and 5.25" for the 9" rim (like I said above), or 5.25" to 5.5" for the 18x9.5.
