having almost blow torched my face off i would suggest going to a garage to get nuts cracked,i got a guy in chessington who will do it if you want
put it this way i doubt you will be able to get enough leverage with stands unless they have been loosened before....chances are under the car you will snap a bolt and will have to drill out and replace.
y pipe to centre pipe notorious for snapping..y pipe to cats no chance, with mine replaced y pipe and cats as one unit infact they are still one unit now