Ed,
Is the internal overflow line bad? That is the only line inside the tank.
If you are only replacing an external fuel line there is no need to drain the tank.
I usually just siphon the fuel into a can. Remove the bolt from the back of the tank. Next disconnect the fuel line from the petcock. Slide the tank to the rear and disconnect the external vent and the fuel regulator return (this one is a PITA). Now lift the tank from the rear, reach under and disconnect the overflow line. You are now clear to lift the tank off.