If you have maintained the original stock "Y". This is the pipe that comes off the headers with the cats and o2 sensors and then merges together to the point where you'd put a cat back on.
If this is the case then just put down Borla cat back system 3" SI/2.25" DO.
But..........
If you split the "Y" and put truduals under the truck then Bill needs to know this.
Hope that helps.
Peace