Charles,
1- Your idea should work. You may not get all of the fuel out of the main tank, though. And, on the ground, you will have to have a minimum level of fuel in the tank to get flow "up over the hump" and then down to the header. I have found that with DE Stubby in level cruise, it looks like the wing bottom surface is level with the horizon. If that is the case for XL, depending on the specifics of your design, some small amount of fuel may remain in the tank, but you don't want to fly at that extreme, anyway. I would be sure to have a positive ram-air pressure vent tube on the filler cap, per plans, and also vent the header tank into the top of the main tank.
2- On DE Stubby, I designed the system to try to have all fuel flow going down, with no place to trap air in a tube, and it did get more complicated than hoped for, and was a challenge to install. DE Stubby has two tanks, one in each wing. Each tank has a filler on top/front/inboard corner with ram-air tube, a small vent tube on the inside wall near the top, two fuel outlets, one on the side at bottom front, one on the side on bottom rear of the tank, and a drain on the bottom surface at the rear, inboard corner of the tank. The fuel outlets each have a finger strainer inserted in the tank fittings.
Referring to the picture, on each tank the rear outlet has a tube going diagonally down and forward, where another tube from the front outlet Tees into it. Then the tube continues to the upper part of the small header tank. The other tank is plumbed the same way. The header tank has one outlet, then a valve going to a gascolator at the bottom of the firewall. (This leaves one critical issue, the clear tube, described below, better not leak, or all tank fuel will be leaked out. So, we keep an eye on the clear tube's condition. Maybe could be better with two valves, one from each tank. But, on a positive note, if the fuel valve is not opened, the engine will run only for a minute or so on the fuel trapped in the delivery tube, so you will have a subtle reminder before takeoff. With a header tank of any size, delivering fuel straight to the gascolator, the engine could run long enough to get into takeoff??)
The top of the header tank has a small clear tube going up to a Tee that connects the two small tank vents together at the top of the tanks. This clear tube allows all trapped air to be vented out of the header and lets all the fuel sources to flow and equalize, and it serves as a fuel level gauge. It has a dual reading gauge strip attached to it; one side of the strip is calibrated to fuel quantity in a ground attitude, the other side of the strip is calibrated to show fuel quantity when the main backbone longeron is level, which is real close to level flight.
Theoretically, with the front/rear tank outlets, all fuel is usable. But, with a flat bottom tank, the fuel level indicated will vary a lot when flying, so I don't continue flying when it gets to around four gallons remaining.
I personally would try to get all the fuel routing going downhill, on the ground and in the air. But, no matter what you end up with you need to check fuel flow capability when the airplane is nose up (we tested Stubby at 15 degrees), and verify the rate and quantity of fuel that flows out at the carburetor inlet tube. Then, with the airplane fixed in a climb attitude, run the engine at full power to verify it really all works.
Hope all that helps.
