[xlr8or]

In the original format they did not use a fuel level guage. They used a pressure guage. The two front holes are for pickups that feed two separate fuel pumps. The lower one goes to the bottom of the tank and the upper sits a bit higher. You run off the higher one until you lose fuel pressure, then switch pumps to get you to pit road or gas station. The idea is the lower pickup has access to about the last 5 gallons or so.
The top port is for a vent. I don't know about the one on the side unless you used that for the vent and the one on the top for a return line or simply put a cap on it.