6 mm is small for an exhaust stud. Access is limited. Before you try to split the nut or cut it loads of Plus gas and walk away for a couple of hours. Re spray leave again.You may get a grip on it with a pair of mole grips or plumbers pliers, grinding/ filing a single flat will increase grip.If it doesn't move and don't try too hard heat and grip again. Better a knackered nut than sheared stud.
