RE: 3 line tanks - 3rd line bend tubing or use tubing with clunk?
This clunky stuff gives me a clunky idea. How about doing the standard line (tubing plus clunk) from tank to carb, the regular vent line bent to top of the tank and then a hybrid of the clunk/bend concept.
Take the fill/drain line and bend the tubing about 45-60 degrees downward and then you can put a very short piece of tubing on it maybe 1/2" to 2 inches only (depending how large the tank is) with a clunk on the end. This setup would never cross the path of the main clunk line and would have the benefit of being able to fully drain the tank. Also the clunk would prevent the tubing from sucking to the bottom of the tank.
What would be really cool is if somebody made a tank with the 3rd line modled in that reaced to the rear bottom of the tank so you could have a built in fill/drain line!