Definitely be careful of the padding. If it is "Open Cell" foam padding, it is EXACTLY what a sponge is. "Closed Cell" foam will not absorb water, but I'm not aware of it ever being used as carpet padding. To test, get a small piece and hold it under water, squeeze, then release. Bring it out of the water, and after letting it drip for a bit, squeeze again. If you get water out of it, it's a sponge. What Charles said applies, all you will get is a rust causing weight problem.
The original stuff used on the floor of the 240 was Tar Paper. It was basically a thin piece of paper with sticky tar on the back side. This was applied in sheets to the floor metal to reduce the vibrations and hence noise. Over time, the tar loses it's solvents that keep it flexible and it turns rock hard. Many of us have discovered that it will crack and break off, and hence we have a noisier car. The Dynamat solution is the best. It is spendy because it doesn't absorb water and it DOES reduce that vibration factor and therefore reduces sound in the car. As posted, it is EEEEEK spensive.
A very low cost but somewhat effective sound deadener is to get a product called Body Shutz. I believe this was made by PPG, and it is still available in true blue automotive paint stores. This is literally liquified tar. You need a special "gun" to shoot this stuff on with, but it has a couple excellent features.
Body Shutz's main use is for undercoating of the carriage. First off, it will protect metal with a coating that is relatively flexible and impervious to rock chips. In coating the metal, it also reduces the vibration resonance that causes sound transmission through the metal, therefore it quiets the car. Since it is a "tar" it also reduces the incidence of rust.
You can also spray this on the interior side of the metal panels, but it does STINK! It remains very, and I mean VERY fragrant for several weeks, however, it will quiet down the metal, and help prevent rust from any water that may seep back there. Just be extremely careful of filling any drain holes, especially in the door.
Well that's my 2¢ worth.