I'm a little confused about what you're trying to do. If you represent your terrain as a single 2D heightmap, then you won't be able to form the same kind of terrain as minecraft, as it is a 3D terrain (i.e. arbitrary placement of cubes anywhere in 3D).
One benefit of voxels is that you can have overhangs, which are impossible with a heightmap like you describe.
In terms of minecraft, a single 2D texture can only provide you with information about a single layer of blocks.
If you really do want to create a 2D heightmap, Vue has excellent tools for doing so:
http://www.e-onsoftware.com/products/vue/vue_9.5_infinite/
Although it is quite expensive.