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.