I've found a picture of the inside of the muffler and it seems to be just a straight pipe, but what is in the big hollow part of the muffler and where are the openings for some of the exhaust to get to the big hollow part?
It's not hollow its packed with some type of packing material. The pipe inside the muffler is perforated and allows some of the exhaust to go into the packing material. 90%+ of the exhaust just goes straight through. The muffler is actually pretty heavy.
I owned a set at one time and they are bad@$$ at idle but anything over idle and the note gets pretty crappy (IMO). I love loud mufflers but the w/ the lack of tone you might as well run SLPs or straight pipes. Combined w/ an o/r X you have the worst sounding mustang exhaust i've ever heard (again IMO).