Think Nik has answered the question of who can carry out the work
The permanent option (should you so choose) could easily be installed using a bypass and control valve, similar to that below, with the muffled pipe running inside and parallel to your existing side pipe, then rejoining so it comes into the side of the turn-out. The major disadvantage I see with this system is that you're permanently lugging about an extra 30kgs of plumbing under the car for 360 days of the year and when you're going racing, kgs = seconds, so you wouldn't even want it there !!
The removable system gives the added advantage of while the car is up (prior to race day), the camber/castor can be set for the track, tyres changed, brakes checked, components inspected and greased, etc., all the things that you do not want to be doing on race day or have a scrutineer pick up