Andy, you are better off measuring the existing pipework on your existing exhaust system.
Depending on the location of your centre box and clearance between the box and the rear wheel, consider using a 45 degree bend; it will reduce the amount of material and weight hanging off the rear of the centre box.
Thanks David,
There's a bracket/support either side of the box so hung weight shouldn't be an issue. More like a problem will be the lack of space to clamp and route the pipe.
I'll try and have a look tonight.....

dependant.