Mark, I've put auxiliary tanks on a bunch of bikes although I've never put a cell on a GL1200A. Its good that you have a target clear in your mind (400 at 70mph), but I think it does help to know what fuel range you have available now. Load up a jerry can on the back, ride to the interstate if that is the test track, fill the tank and jerry and explore that lower 1/8th. Use the jerry can when it coughs and now you know your starting point. The aux capacity and style (pillion seat, pannier, rear rack etc) can be sorted out when you have a final total volume in mind.