You will have to remove the muffler first. Then lay the muffler on the floor with the exhaust side pointing toward you and also on the top half of the muffler.. Straight up from the exhaust outlet in the top right corner, cut out about a 3 to 4 inch square hole. You will be right beside the internal cat. Now take you a large bit or one of those tapered bits and drill you a bunch of holes right beside the internal cat. ( I think that I had about 8 each of 3/4 inch holes in mine) Now you just weld the square that you cut out back in the hole you cut out. I plan on running mine on Sling II for a good while. Right now I am not planning on putting the Welter Dual back on.
I do have a picture but do not know how to put it here. Here is the number if you know how to do it. It is on here somewhere 0531190900.jpg This is a picture of the cut open muffler