So, we are have run into a bit of a snag. Our issue is that we had originally started out with a voxel system. Once one of our coders figured out how marching cubes worked we started to play with that... now we want a system that will let us draw cubes for certain block types and marching cubes for the terrain. We cant seem to get hard edges like a cube though.

http://www.paulbourke.net/geometry/polygonise/

Is the algorithm we are using now. Any thoughts on how best we can obtain this result?

There's a technique called 'Dual Contouring' that extends upon the Marching Cubes algorithm. Aside from weight data on grid corners it also takes tangent vectors on the grid field into account, to allow for better reconstruction of edges.

The Procedural World blog has a post that explains it well.

Thanks, that has some really great information.