I would go with an aluminum tank to save on weight. Just adding 20 or so gallons may not cause any issue with the emission controls as long as it is plumbed right. Adding more fuel than that might require an additional charcoal canister. That is where the Transfer Flow setup is real nice because it comes with everything you need. They are high-dollar but in many ways worth it. Not sure if they have anything for the F150 yet but if I remember correctly they were at least working on it as well as a replacement mid-ship tank. If you go with them I recommend the Trax II system with the screen. It takes the guess work out of knowing how much fuel you have and if the pump is on or not.
One of the issues with gravity feed is with newer vehicles, if the ECM doesn't see the sending unit (fuel gauge) move within X amount of time driving either a code will be set or the fuel gauge goes to E until next key cycle. You would have to eliminate the OEM fuel neck completely as well as possibly capping some vents and tying some into the aux tank. To use a solenoid along with gravity flow you have to know that the fuel has no debris or junk in it or the solenoid could stick partially open resulting in at least a big mess or worse. Using a pump is usually the best way as long as the fuel pick-up goes above the highest fuel level point in the aux tank.
Using a transfer or refueling tank with a pump and fuel nozzle is probably the easiest but you will have to stop to refuel. They are handy if you have other equipment to refuel as well. In some states this may be the only legal way.
Hope this helps you.
__________________ 2012 F150 EcoScrew 4x4 XLT 157"wb MaxTow LEGAL DISCLAIMER: Anything I or anyone else says about my vehicle on this website or any other website, pertaining to modifications, is only to gain acceptance from my/our peers, and does not actually represent anything actually existing on my vehicle, and thus, cannot be held against me in any issues, i.e. warranty claims, that may arise.