Every panel got 'Silent Coat Sound Deadening' sheets put on them - they're a polymeric based damping material that absorbs vibrations. The van was notably less boomy. and worth every penny.
Insulation that was used was 7mm closed cell 'Dodo Mat Thermo Liner Insulation'
Because I'm doing the flooring in a way which overlaps the side step, I'm having to run the electric cables that I will need under the step first before the flooring is laid.