I've used a variety of aftermarket generic mufflers.  Never had a problem
with the mufflers.
I found that you have to spend the time to get all the hangers and brackets
in place.  Nothing less than what was originally there.  Last muffler I did
on a syncro, I welded up some additional supports and brackets.  ALL the
original (and more if you can) brackets and hangers have to be in place or
the whole exhaust system will evenutually crack and fall apart.  Vibration
will fatigue the muffler joints, collector, headers and so on back up to the
heads.

Gary Lee
http://www.telusplanet.net/public/gary2a/rack/vanagonrack.htm