G'day Mate,
Your clunk should swing freely, when you hold the tank vertical, out of the plane, of course, adjust the clunk so that it does not touch the end of the tank, at all, & can swing up, down & sideways, without restriction, then give it about an 1/8" clearance, from the centre of the tank, not the bottom, to allow for the fuel tube to grow, don't worry, it will seem a long way from the bottom corner of the tank, but it will still use all the fuel, if the tank height is correct.