I love your use of the kaleidoscopes! Very interesting placement! And I love the way you incorporated the original fabric. Your construction ideas are really good.
If you are going to add length I would add the same amt at top and bottom. That way your center block will still be in the center and it won't look so much like you added on to get more length. It will be more balanced looking. So if you are going to add 6" more then add a 3" strip at the top and a 3" at the bottom. I would add the same fabric you are going to use for the border. It will blend and not be as noticable that you added more to make it longer.