I went through a similar saga (still not done) with my rear seats and shelf.
http://forums.pelicanparts.com/porsche-911-technical-forum/643329-72-rust-repair-long-overdue-update.html
I just ordered up a bunch of dynamat. They make two base butyl materials....a regular one (xtreme) and a lightweight one. I plan to use both depending on how much I think that area will transmit vibration. I will top that with 1/4" dynaliner closed cell foam, then an appbiz RS carpet kit. I hope to lose weight over the original factory setup...