TDi exhaust onto VR6

OK, this is going to sound strange, but i'd like to know the diameter for a TDi exhaust system, and what would be required to fit this to a VR6 from the cat (de-cat in fact) back?

At the moment I've got an immense custom-fabricated system, with a single box from the cat back - but I'm having a rethink with how I want the car to look, after some real indecision, I'm working toward a kind of GL-spec sleeper. I think a couple of weedy looking twin down-swept pipes would fit the bill perfectly

Also, I'm trying to get an idea how things would sound, with a decat + standard VR6 mid-box + TDi backbox, versus a decat + TDi mid-box + TDi backbox; although I suppose there's only one way to figure this one out... I'm thinking the quieter the better at the moment.

Jim