Thanks to all who responded to my call for help - the solution was simpler
that initially thought:

Mike Catlin offered me a used 'spare' crossover pipe that was missing one
stud (and the threads in the hole) on the right side (connection to right
heat exchanger). Since this was the side that my own pipe was faulty (quote
from garage: 'at least two of the 'bolts' = studs stripped, you need a new
crossover pipe') I decided to take the time and everything apart to check
my options. 

My pipe had all three studs in place (just more or less stripped in the
area where the nuts should be) and looked fine otherwise so I took it to a
machine shop to have the old studs removed and new ones put in (they had to
drill one out: $ 25). Got new bolts and nuts for the left side flanges ($
10), reinstalled everything, and - lo and behold - the increased noise is
almost gone (Mike, I did not use the pipe you gave me; do you want it
back?). A new muffler and tailpipe should take care of the rest...

I wonder if the garage (they claim to be VW aircooled specialized) should
have known better, and just did not want to suggest an easy (cheap)
solution???

Marcus
'80 CA Vanagon Camper w/ 2.0l FI