This worked out great! I rode it a long way today, and burned around half a tank. It didn't slosh or spill, and the ride was stable and comfortable. I felt confident with the extra fuel, so I could ride WOT instead of babying it to save gas.
This tank has made this build a success. 1 gallon of fuel weighs about 8 or 9 lbs, and is very noticeable on this bike, but with it centered over the rear axle, it minimally effects steering, and is lower to the ground by far than a HT style top tube tank. At 100mpg, I can just get on this bike and make a lot of short trips without dealing with fueling.
This works great. I like the thumb-indent cap best. (US Plastics corp makes the tank and lid. They offer several lids, and don't forget the tank-neck drop-in fuel filter. I wish I'd bought it. Cleaning the tank is not too easy.)