Since installation of most catless downpipes will require some form of welding at the downstream end of the exhaust, have you checked in with Josh at mandrel exhaust systems?
These folks sold me the pipes necessary to weld together my own custom exhaust system from the end of the exhaust manifolds all the way to the tips: from the end of the exhaust manifolds to the tip with all the necessary flanges and hangers I wanted.
This was for my 5.0 engine which has the same exhaust config as the 3.8 and 3.8SC so I know they will have the template. My pile of parts was cheap but if you plan to tie into your existing exhaust (say at the Y, your system could potentially be even cheaper).