Mattress in a conversion van width wise seems to be a close call.
Typically you only get a measure between the wheel wells.
After that the shape of the van makes it not so clear.
For width wise what is the tallest person height to sleep comfortably?
Normal van like a Ford Transit or Chevy Express....
Typically the widest part of a van is about a third of the way up (although this is usually only very slightly wider than at floor level) and they usually narrow a bit towards the roof.
I happen to have the floor dimensions of a Renault Master (similar size to a Transit). The floor is a little over 1700mm (68 inches) wide max. I'm not aware of any standard body vans which are significantly wider than this . However you may need to knock a bit of this to take account of any lining and insulation on the sides.
So based on that 5'8" would be the absolute maximum for a horizontal mattress but realistically probably a bit less.
I have some experience in working on industrial van conversions and as you say it is not trivial to work out exactly how much clearance there is for any given installation even if you have a van there to measure. However, in my experience, the larger panel vans (ie the ones not based on a car floor pan) tend to be fairly square and don't generally deviate from the floor dimensions by more than an inch or so until you get fairly close to the roof line.
Typically the wheel arches intrude into the interior space by about 5 or 6 inches per side on a large van (apart from double wheel models obviously).
If you get the chance to measure a van the distance between the horizontal stiffening rails about a third of the way up the side should give you a decent idea as this corresponds fairly well to the max internal width once a lining is fitted (typically 1/4" plywood) minus a bot extra for any insulation/trim on top of the lining.
As Chris says in his answer "widest part of a van is about a third of the way up". Everything in his answer is correct.
I have slept many nights in a homemade conversion van. If you are sleeping alone, then sleeping diagonally gives much more length. Useing Chris's width of 5'8' (68 inches) a width of 48 inches, and a Triangle calculator we find that the diagonal length is 83 inches (almost 7 feet)
Another consideration is heat or rather lack of heat. Metal vehicles like vans, shed heat quickly. You go to sleep stretched out diagonally on a warm evening and you wake up curled up trying conserve body heat. From experience getting up at dawn, it usually feels warmer outside the van then in it.
Putting a bed above the wheel arches is common, with storage underneath. The width you get is similar to the width behind the wheel arches, and that's often specified (it certainly could be found for the transit when we were looking). But I'm well over 6ft and when we bought our transit for conversion we looked hard and found there was no sensible sized van that would take even a 6ft bed across with insulation. You would end up using a lot of length to make a comfortable bed diagonally across the van (you should start with how tall you are and then work out how big a bed you need rather than the other way round). If you're putting in a double bed you can save a little by sleeping staggered.
In fact the insulation doesn't make much difference because of the structure of the walls.
We went for a slide out design sleeping along the van and fitted a decent kitchen plus a toilet cubicle in a medium wheelbase transit. The bed goes away easily but you can cook with it out. A major reason for this design is that there's a full length gangway up the middle in which I can stack kayaks. I've had 4 in there with room to spare. Or if can take bikes inside for when they're at risk on the rack outside.
I recommend going to a place that sells the sort of van you're considering, taking a tape measure. We looked at a few before settling on the transit.
