Consider this a proof of concept.
You can follow these directions precisely, but depending on the initial placement of your 3D Cubes your outcome may be different.
Play with the Transform and other Filter values to fine tune your 3D object
The 3d transform tool (if not installed by default) can be found on the Resources & Extras CD. If you don't have the disc, a quick Google search for "3dtransform.8bf" should do the trick. Install like any other filter, just drop in the Filters folder.