Paul,
As long as you have enough space to fit that tank in there I can see no reason to make it any shorter. It appears that the total length of that tank is around 10-11", I would suggest that you simply make the rear bottom of the tank taper upward to prevent the fuel from pooling in the back and make the clunk line about 7-8" long (with a length of brass tubing to keep it from whipping to the front of the tank under harsh decelerations). I think it is impossible to keep the clunk in a large tank covered in fuel under all conditions but as long as you have smaller tanks (UAT, or some other type of air trap) between the main tank and the pump it is not going to be a problem if you get momentary burps of air. Just be sure that when the airplane is experiencing positive Gs the clunk is always submerged.