Those kits are the best chance at getting a flush bolt out clean and worth a shot. That said, I have had mixed results using kits like that on small hardware like #6. Might be a good thing to have a Plan B. Depending on where the post is located, you might be able to live with leaving the screw in place and drilling a pilot hole directly adjacent to it to mount the post. In the end the post will only move a tiny bit and will cover the broken screw.