And an adjustable wrench is a piss poor tool to use when trying to loosen a nut, especially one that has obviously been over-torqued. You're asking for a slip-off, testicle busting experience using one.
When you re-install that castle nut, the book calls for 85 ft-lbs torque. If you can't get a socket and torque wrench on it due to tailpipe interference, at least get you a good box end style 27mm(I think) wrench to fit it instead of using an adjustable. You should be able to stop short of using a cheater bar to re-tighten. Then put the cotter key back to insure the nut does not loosen.