We run a Dispatch Combi (5 cu m volume and 2m tall so probably at the smaller end of your van ideas) with a rear bulkhead and bike shelf. With front wheels removed, saddles full height and bikes vertical this gives around a 1 foot gap underneath (so maybe enough to poke legs into if sleeping lengthways). With dropper posts / post removed you could gain some more height. We now get 4 full size bikes on that shelf.
Also be careful about eating into useable storage space when insulating and panelling out – e.g. look where the handlebars poke in the photo below.
I have a T4, wanted to be able to get 2 bikes in with the wheels on.
I found that 3/4 beds were too wide, so looked at custom beds. Then found a Bongo 3/4 bed was the width I needed (90cm). Bikes go down the side, ideally if 2 bikes 1 or both need the front wheels off but they do fit with some faff without removing the wheels.
Load of space in the back for all tools/mobi washer etc, and then have a single unit opposite the sliding door for camp/cooking stuff stuff.
you can get them on amazon for around £15-20. Can use it for storing anything but yes it will take my helmet and stuff. you can make it as tight or slack as you want.