Walker and Bosal make Cats that fit.  The Walker is part #15612.  It fits
pefectly and has the hole in the correct place for the O2 Sensor.  The
element is slightly smaller than factory but the flanges are a perfect fit.
 The inlet will accept either the donut or gasket depending on your inlet
pipe.  This cat can be used for all Vanagons including air cooled by merely
pluging the O2 sensor hole.  Cost should be about $150.00.  The Bosal is part
#099-922.   Flanges are more universal with inlet being a swivel.  This cat
is a pancke (oval) design that and the bolt holes are big requiring 10 mm
bolts or washers.  Includes O2 sensor plug for use on earlier vehicles that
have sensor in pipe.  This one costs about $140.00

Hope this helps all,
Dennis H.