I did mine two years ago on July 4th morning.  I bought OEM muffler, cat, OXY
sensor, gaskets and bolts.  Yes, very expensive from the dealer.  But it was
a relatively easy job.  My original OEM system had developed a rattle in the
cat.
Three nights before my designated morning I sprinkled penetrating oil on all
bolts that needed to come out.  I waited till my Vanagon was parked for the
night and the exhaust system was hot.  Then I applied the penetrating oil.
Repeat this procedure every night prior to D-day.  On the designated morning
all the bolts backed out easily with an extension bar.  No need for a hack
saw or cutting torch, no rounded off heads.  I was pleased with the ease with
which they came off.
I live in sunny South Carolina so rusty brackets are not a question.  My
brackets were in perfect condition.
I bolted the cat to the muffler and inserted the new OXY sensor prior to
installing the combo.  It is easier to bolt the parts together before
installation in the vehicle.  I used my floor jack to support the combo while
aligning the bolt holes.  The combo was quite hefty.  One advantage to
purchasing OEM replacement parts is that they fit PERFECTLY!
The entire project took me a full morning, I started at 7:00 AM and finished
clean up by noon.  One could probably do it faster but I was in no hurry.
Good luck with your project.  Keep the list informed of your progress.

Dennis Lockwood
91 Vanagon Carat
Original Owner