I am using sunburn 2.0.8's editor, and it gives me an item's bounding box in the 3D World. I've tried doing Box box = new Box(obj.ObjectBoundingBox.Min, Matrix.Identity, obj.ObjectBoundingBox.Max);
I've just started with this, but I *think* the problem you're having is that the framework BoundingBox is defined by 2 corners (min & max) so that the size of the box is implicit. The JigLibX box is defined by a point and then explicit side lengths. In both cases the args are all vector3s but they have radically different meanings.
If I'm right you need something like:
new Box(obj.ObjectBoundingBox.Min, Matrix.Identity, obj.ObjectBoundingBox.Max - obj.ObjectBoundingBox.Min);
Hope this helps.