For the record, and those contemplating doing something similar.
i went for a test drive in a MGB GT that had been re built by Frontline Developer and fitted with modern engine and gearbox and all sorts of other goodies. I wanted to see if the rear suspension mod was worth it.

When showing me round the factory they pointed out the sound deadening (I can look back if necessary to see which brand). It had obviously made a good impression on me as the car was very quiet.
Flushed with that thought I lined a MGC before I had to sell it and also added some foam insulation too.
As far as I can remember it made very little difference compared to the MGB that I have now that has felt like material under the carpets.
In my opinion it is more of a science than one might think and getting the right materials in the right places is very important.
Stopping vibrations by adding pitch backed type materials might stop vibration but to really dampen noise down there is probably nothing much better than the “dead dog”.
Years ago (decades) I fitted some old foam backed carpet into the load space of my small van. That made.a fantastic difference in audible comfort.

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