I have fitted the half kit in my Westy. I did this when the van was completely empty. I decided against the full kit as I didn't see the point of placing under the floor.
I placed the sound deadening under the front cab mat, walk through section and rear cargo area (running it down in one complete piece to the lower floor)
I also stuck the self adhesive material behind all panels and over the rear cargo area.
Lastly I fitted the egg box material on the roof behind the wooden head liner.
You should be able to do a lot of this keeping the majority of the interior in place. Having owned vans previously that weren't fitted with any sound deadening the difference now is unbelievable and well worth the time and money doing it.
If you haven't already bought the kit, I wouldn't bother with the full kit.
Mike