Voxel Engine Coding Blocks

Hi. I am using a voxel engine I found on YouTube(http://www.youtube.com/watch?v=qOeaG2Q686Y) and I have worked out how to add blocks, and generate them in the world, but how would I go about giving one block a custom code. In this case, I would like to add a furnace that has a GUI, and the front texture changes when something is in it. I don't want a whole piece of code or anything, but snippets or pointers in the right direction would be very helpful.

Thanks

AshJack