I started putting down the floor system tonight after work.
1/2" x 3" plywood strips as sleepers or joists.
1/2" x 9" foil faced foam strips between sleepers.
This layout puts the sleepers at 12" on center which will work well for the sheet products that go down next.
I used PL500 a polyurethane construction adhesive (non-toxic) to glue the sleepers down to the sheetmetal van floor. (Not a yard sale- heavy items for holding the sleepers tight to the floor while the adhesive dries/bonds.) Next will be a 1/2" layer of foil faced foam over the sleepers and foam strips to provide a thermal break before the plywood floor goes down. I will screw the plywood floor through the foam and into the sleeper strips. This way I will not have any screw penetrations through the van's metal floor. The complete floor system will be 1 3/4"- 1" foam and 3/4" plywood underlayment plus some thin garage floor tile or similar. I have not picked that out yet but that will come much later- after cabinets etc. The floor thickness may not work for some vans, but this one should still have 6'-4" or so of headroom after its completed.