Well, I bet it would be a lot of work and expense to raise the roof. As far as finding a bigger box, there are lots of truck salvage yards that stock used boxes. There's a newspaper called "Truck Pages" or "Truck Papers" I think. Lots of salvage places advertise in that. Most heavy truck dealers and repair places have a stack of the papers. They're free. I would guess they have a web site too but haven't looked. A handful of guys have put a used box on their truck frames that I've seen. It doesn't look like a huge undertaking to do. Good luck.
'03 Freightliner FL112, 295" wheel base, with '03 United Specialties 26' living quarters, single screw, Cat C12 430 h/p 1650 torque, Eaton 10speed , 3.42 rear axle ratio