The problem with the Hiace is hardly anyone buys them new. You won’t get an 8×4 in the SWB so its the LWB only on that count, and from a selection of very few, only an small proportion are LWB.
on the 8×4 criteria only a big question is how you want that sheet to go in. Increasingly van manufacturers are only accommodating them flat on the floor, a rearward sloping bulkhead means you couldn’t have a board on edge strapped to the side of the van. That discounts lots of current vans for me as I’d need to totally unload all my kit each time I wanted to put a sheet in or take it out.
If you’re happy to carry sheets flat then smaller vans would be maybe be
VW transporter I think swb takes a sheet flat ( you can just get an 8×4 in on edge in the LWB)
Merc Vito (on paper sheets will go in flat in the LWB or on edge in the rarer XLWB but they won’t go in the compact)
New shape dispatch/scudo or peugot thingy – the LWB would allow sheets flat on the floor only
The renault trafic and similar will not quite get an 8×4 in flat (2.4m so plasterboard will go in plywood won’t) the LWB will take a sheet on edge.
The LDV vans can take sheets flat but the bulkhead leans back higher up, so a taller stack of sheets would go in
After that its proper transit sized vans – tranny, swb sprinter, boxer.
Verify all that with a tape measure at a dealers though – manufactures figures can’t be trusted and different types of bulkhead may reduce the available length at floor level.