One advantage of getting a complete donor Disco is that you can use some of the soundproofing in the Series, and if you are lucky enough to get a 'posh' Disco, there will be a bulkhead soundproofing mat as well!
Here's a (not very good!) pic of the bulkhead soundproofing - the only pic I can find from before I fitted the engine:
We also find a blanket over the gearbox helps a great deal, under the tunnel:
I'll add more pics when I've got the floors back in my Carawagon, but basically, we use as much of the soundproofing from under the carpet of the Disco as we can.
Cheers,
Robin.