My local German Indy has replacement cats he just welds back in - simple $250 fix!
You'll probably find the 4.2 exhaust will only be compatible from the Cats back (assuming it has the usual twin offset pipes coming from the twin cats), and may be larger diameter (2.25" vs 2"). A bit of custom work may be required to make it fit.
I wonder if its the left Cat that's rattling? This is a common failure point on B5 V6 exhausts!
2001 Audi S8 - Solar Sunroof, heated front and rear seats (just what you need in Australia!) Tinted side and rear glass, RNS-D, XCarLink, Bose, clunky old phone in arm rest!
1997 Audi A4 Quattro - 5spd man "Hamilton's Club Sport" - will miss the old girl!