I've used spray-on soundproofing in areas where moisture cannot get trapped between the sheet meatal and the spray product. Areas such as inner door panels, underneath floor carpeting, etc. seem to be good candidates.
Also, if you ever find yourself in the salvage yards, grab as much of that under-dash soundproofing as you can. An extra layer never hurts. This advice courtesy TCCBass here on mercedesshop...

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Matt
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Grey/black '84 300D, 194K