i haven't used nut splitter's...but..have you tried to remove them when the engine/exhausts hot and therefore the metals expanded?...if that don't work maybe try using a dremmel type tool to cut them off?...

if there that fecked maybe the threads on the studs are too...new studs and bolts might be the way forward..then you could grind the nuts off anfd replace the studs..i have used threaded bar and londg bolts in the past..hth