Please note that the screw that holds the mag cut off also holds the mag tube. They are threaded very tight. You will have to drill a new hole in the mag tube for the screw after it is installed. This should be done on a drill press to avoid damaging the screw hole threads. Remove the cut off screw and cut off. Some heat may help. Heat the tube not the receiver. Other items like a padded vice, penetrating oil etc etc come to mine. If you don't intend to salvage the old tube a pipe wrench is a good tool for the job. Good luck. I would have a smith do it.
regards, Darryl