The other exhaust has to have the cat removed so that they match up so my next question is can this be done whilst that part of the exhaust is still connected to the pipes from the engine? I guess you have all noticed I'm not great with anything mechanical but I'm giving it a go
The more expensive, sharper cutting device should work better, but the exhaust material is indeed pretty tough. You will have to remove the header to bash out the remaining CAT. Use a long, flat screwdriver to break the bond between the substrate material of the CAT and the wall of the exhaust (go round the material in a circle), then put the biggest screw you can find into the middle of the CAT substrate and pull it out like a lollipop. Provided you've released the bond between the substrate and the header wall, you should be good. Remember to clean up the inside of the header nicely and make sure all loose material is removed.
