Most aftermarket gas tanks today have a sealing gas cap, and the fuel outlet in the top with a pickup tube going down to the bottom. They also have a check ball in the vent tube to prevent all the fuel from draining out the vent if the car goes topside down.

This is a DOT requirement on all new cars. I'm not sure if it's required on street rods, but it's very possible your state inspection could reject a car with a bottom-feed gas outlet. Even if the state doesn't require it, it's another safety idea that should be considered.

http://www.tanksinc.com/index.cfm/pa...prod/prd82.htm