I don't believe the size is the issue. But you are trying to mount an oversized runflat (read very stiff sidewall) on an 8.5 inch rim. Most tire places struggle getting the 245/45's on. I can't imagine the 275's getting on there without a fight. If they can get them mounted there is still no corresponding set for the front as they don't make another size in 18's that fit the front. If you don't mind running a mismatched set of tires you may be fine.
I have GSD3's in 275/40 in the back. They bulge a lot. I don;t see how those uber-thick run flats are going to fit. Even if they do, I'd wager your rim would be scratched to hell and back getting them on/off.