You may want to consider not creating a one-piece turbo to cat pipe. If you do then if you ever change turbos you're most likely going to have to have another custom one made. What may be easier is to have a custom downpipe made and a custom turbo outlet made, so that if you ever change turbos you just need a new turbo outlet to mate up to your existing downpipe.
If you never plan on changing turbos then disregard everything I just said
