Thanks, ISBRH seems to be the good way. I successfully made a block using ISBRH and rotated the texture on the top. Now I just need to adapt the uvRotateTop to the metadata, I hope I'll succeed
About the furnace: like Draco18s said, the only thing this block does is putting the front texture at the right place. The top texture never rotates (you can verify on the pumpkin block which works exactly the same way, the stem drawn on the texture doesn't rotate).
EDIT: Done! Thanks for you help! If you want to check the sources, go ahead: https://github.com/KillerMapper/roadstuff
Note: I just set the git earlier, and I still have lot of test files in. I need to clean the mod. Check TestRotateBlock.class, which uses RenderAsphaltLines.class to see how I did the rotating texture.
Thanks for your help!