Moroso has a "cherry bomb" muffler that does NOT use fiberglass and is design with very little back pressure. The inside devides the exhaust path into two parts in a spiral. ($54)
Many glass pack mufflers spew out particles of fiberglass while running. You can see the particles glistening in the sunlight. You will end up breathing that, or have it all around your garage, driveway and neighborhood.
The mufflers that just have a perforated tube inside (many holes), that is straight through may not muffle the sound enough for street use.