I recently replaced the after  market muffler on my 84GL with a German
muffler from Bus Depot.  I also replaced the brackets that hold the muffler.
The first time out the van was so much quieter and smother I had to check
under the deck lid to make sure the old engine was still in there.  To get to
the point all these years I've put up with the droning  and vibration at
around 65mph and above which I thought was caused by worn motor mounts and an
engine with 120,000 miles on it was cured by a new muffler and brackets.
-dieter