I had Reflex brand bed liner sprayed on the inside firewall and floor of a 61 F350 with a Detroit diesel. The Reflex is pretty rubbery once cured, not hard or tough. The old nasty carpet did a better job than the Reflex, and the Reflex when warm smelled nasty for a week of driving (cross country). Windows open, 75 mph and still get a head ache.
The folks who installed it said it'd do a good job of sound deadening.... that's BS, it may have done something but it wasn't worth the effort. Later on I was having something coated in Line-X and asked them about sound deadening... they said no surprise that the Reflex didn't do much, apparently it doesn't have much ____ in it and line-X does (not sure what component they were talking about). They said they do use Line-X for a sound deadener with great results, inside work vans and things like that, and no smell at all. I don't have any first hand experience with it.
The plan for sound and insulation in my truck is use some stick on sound deadener in smaller pieces on the larger flat panels, cheaper than plastering the whole thing and almost as effective. Then use some heat insulation over that, probably this
http://www.lobucrod.com/
Might have line-x sprayed in as a floor mat over some plastic sheet or something so it can be removed easily. Basically a custom fit floor mat.. not sure if that'll really work though.
Some more sources/info
http://www.sounddeadenershowdown.com/cgi-bin/index.cgi
http://www.quietride.com/
http://store.secondskinaudio.com/
Grigg