dude if i was that guy at the muffler shop i would do it for the simple reason you'll be back to spend more money on getting legal again
lol, you firstly say to the guy at the muffler shop that you have a race car in the build and this car i have brought here today is the same chassis so you can use it to prefabricate a exhaust system for it.....
secondly, a full straight pipe system will probably cost you the same if not less then modifying your current one.
thirdly, when you do get defected, atleast you can go home and bolt the stocker back on.
considering the average price for mild steel 3 inch 1.5 ml wall is 10$ a meter plus 30$ a bend.
so your looking at 200 dollars max for a whole stright pipe system turbo back....(not including flanges. )