RE: Header Tanks
You do want a clunk in the header tank for a few reasons. First, you want to be able to completely empty both tanks for storage, so you want a clunk at the bottom of both tanks to empty them. Second, if you can't see the header tank (e.g. if it's enclosed inside an airplane fuselage), you'll want to be able to use all the fuel in the header tank if you happen to fly too long and empty the main tank. Third, a little bit of air will end up getting into the header tank, and you want to be sure you've got a clunk in there to pick the fuel up from the bottom, and not get the air that will bubble up to the top.