Headaches are a good thing, you are detoxing. The best juice would be home made organic apple juice. If you get papaya that would be even much heavier. I finished my 22 day fast and now on juices for two days.
Keep it up it will clean your body. Do you like vegetable juices? Make sure you do not mix fruits and vegetables.
When you have a headache, just lie down and breath deep. Breathing deep also detoxifies your body.