New (used) muffler.. which way is up?

[Outrageous]

I have the same issue on a 23 inboard restoration project. The boat was a basket case when purchased but the muffler that came with it looks like what you have in the picture. Being below the floor I would assume that it is full of water until on plain when it would continually drain itself. I would imagine that the original configuration had this some type of mounting or attaching to the hull to eliminate vibration. Anybody have any pictures of an original setup or knowledge of how they work? Mine has two 4" inlets and one 4" outlet.