I think an under 10,000 GVWR cube van is behind the 8-ball on weight capacity, with or without an attic. These vehicles are better suited to personal moving than to the type of work we're doing. Imagine a vehicle which can haul less than 3,000 lbs, but which has the wind drag and fuel consumption of a larger truck-- in my opinion, this is a no-win.
I had occasion, a year or two ago, to drive one of these things. The volume was greater than a standard van, no two ways about it-- but when I saw the payload rating stamped on the sidewall-- this was a rental-- I had a hard time believing it. My 2500 standard van can carry more weight than that 3500 cube van could.