Black Forest has a great product they use for sound deadening in cars that owners want quieter. This stuff is quite remarkable, I did my 190 SL with it. It's basically a layer of lead covered on both sides with foam. Very pliable and molds to whatever you're working on. To get the idea of what it is, think of the cover the dentist puts on you for taking Xrays. It's that heavy and substantial. Works beautifully and really deadens the noise contributing to the legendary MB vaultlike sound.
Here are pics of my original floors, the lead being installed and the finished result. I didn't do the center tunnel, but may.
As you can see, the rear well is the most difficult with all it's different contours. I wanted to keep the lead one solid, removable piece rather than section and glue separate pieces in .