I used to think that the fuel line should be able to move towards the front of the fuel tank, when the plane is in a nose down attitude. But now I am not sure it needs to. The Gee forces applied to the plane, and fuel tank tend to force the fuel to stay at the rear so the clunk remains in the fuel. I think even in a stall-spin manuever, the fuel tends to stay at the back of the fuel tank as the plane is falling nose down. So I am not sure if the clunk needs to go forward anymore. Maybe it is a myth that has propagated around all these years.  I think if one really needs the clunk to go forward and they are using the larger fuel tanks, they only need to use a heavier clunk to overcome the more stiff fuel line.