To put it simply, there is no off the shelf y-pipe from a GM truck that came with an LS engine. The GMT800 trucks have a driver side front t-case output so the factory y-pipe crosses over the passenger side, merges and goes down the passenger side. Which happens to be in the way of a Sqaurebody with it's passenger side t-case output to the front.
Build one or have an exhaust shop build one. You will be able to maximize the routing to fit your truck better than trying to adapt the wrong pipe into the mix.