As a base for the interior, I decided to put in a ½” thick piece of plywood on top of ½” rigid insulation board. I started to fill in lengthwise indentations of the metal floor with strips of a thin insulation material, to maximize the R-value.
Subsequently the insulation boards were applied. This applied only to the rear half of the van, since the other half still had the original insulation installed. I only had to fill the areas under the former rear chairs, which were left bare.