OK guys a few questions answered ,I will be joining the other exit point up with the pipe but I need to do that work as I don't have the tools at home to scallop the holes and pipe.once all welding in I will pack the box back up with all the sound deadening material to prevent any droning .
Hodroyd, yes I could dispense with the box completely but that would have involved hanging brackets being made and a lot of eyeing up and probably tacking it up on the car before welding to get the alignment right, and all on the floor as I don't have a ramp. This way, a couple of axle stands, unbolt the originals, bolt the modified ones on and Bobs your uncle.