I have done a few of these when I was at christo's shop. I just removed the Y-pipe and cats. Welded the whole Y-area with Mig welder. I did shove a 2x4 in between the pipes to move the pipe out wards a bit, so when the pipe cooled it did not move in with the weld. It only took a few hours to make it work right. You may want on had the 2 upper exhaust gasket and the one one bigger exhaust gasket to replace when you go to install it.
Good luck with this, later robbie