il be coming from the manifold, but il source the front pipework myself
excellent description mate, very helpful.
il be using 2.5", and trying to aqquire a pipe bender for the 90 bends + then welding in flanges, i want to keep the cat/de-cat swap do able for MOT.
or welding it all up like you have using pre-bent steel
were'd you get your flanges ?
cheers 
