fuel pump push pull question school me

Even with a standard tank, the fuel is sucked up high before dropping down to the pump since the highest point is near the highest part of the trunk floor. The fact that your tube enters thru the top of the tank is irrelevant. What matters is how high your fuel tube goes. It seems like you run it up much higher than required. Why not put a 90 deg elbow right off the top fitting to keep the tubing lower?

Re how high could the tube go? If the pump could draw a perfect vacuum (unlikely), it would be limited to a certain height by physics, which varies with fluid density. That is why mercury is used in traditional barometers, since a water column would go to the next floor up.

Re diameter of the fuel line, there are trade-offs. A larger diameter gives less flow drop, but if too large it lessens the siphon effect. Someone mentioned the weight of fluid causing a slosh-back under acceleration. Most carbureted engines were designed with 5/16" tube, so probably sufficient. High-pressure EFI pumps bypass fuel around the engine so higher flow, though the latest vehicles must bypass within the tank (1-way trip to engine, EPA mandate). The current Corvette has a 3/8" line, but not sure that was required.