da Rock

There is a good reason to have the pickup line to the carb with a clunk on it. The same reason applies to why that line should go to the back of the tank, not just halfway. The fuel line that runs to the engine's carburetor should reach to within 1/4" of the back of the tank and needs to be clunked.

There is really not much reason to have the fill line clunked. The fill line is only in use while filling and is stoppered when the tank is full. Why would it matter where the fuel pours out inside the tank? The fill line can stop just inside the stopper. It can go anywhere inside the tank but why bother. Once the tank is full and you stopper the fill line, it ceases to affect the way the tank works or the engine runs.

You need a line to indicate when the tank is full. That line usually runs to the top of the tank so when the tank is full, it overflows. It's often the muffler pressure line because it then doesn't need to be capped or otherwise touched.

With those three lines, you simply unplug the fill line, hook it to your fuel pump line, fill the tank, plug the fill line, and you're ready to crank the engine.

If you put the tank below the carb, you're going to have engine troubles. If you must position the tank below the centerline of the carb, the usual solution is to run a small secondary tank that is close to the engine and pressurize the main tank so that it's fuel is forced upward to the secondary tank which then feeds the engine.