I used some Dynamat on my 1800ES in the door inner panels behind the door trim. Made a huge difference in how the doors shut - now with a nice thud. Bought a 300-400 mm wide roll of foil-backed self-adhesive tar roofing flashing at Bunnings and used that in the floor of the 1800 & 145 and it seemed to help. Also added a layer of the shredded up re-constituted cotton stuff with a foil backing from Clark Rubber. I agree with James on the transmission gear lever hole in the tunnel - if manual do as much there as you can to seal it up. There was a foam piece (part number 1035596 - see pic) originally but not left on many cars as they degrade and get lost when clutch is swapped out. I’ve heard people using old seat foam, or pieces of sound-deadening mat as an alternative as the foam but is pretty pricey. No amount of insulation will help if your exhaust system is crap and is not properly mounted and rubber-isolated from the body (ask me how I know!) IMHO anything that makes the car more enjoyable to drive is worth it - if quieter is better, then by all means do it. I used to like a loud exhaust...but past that stage now LOL! :)