If the leak is in teh short metal tubing that is immediately aabove teh top of the fuel tank, those short line sections are part of the fuel sender assembly. If the sender has rusted through and has a leak, the entire sender assembly must be replaced. it is not repairable if this happens; the fuel travelling through the fuel line out of the tank is under pressure, and if the metal has deteriorated to the point that there is a leak, any attempt at repair would be unsafe and not likely to succeed. This can become a safety issue if the leak gets larger or if a repair attempt does not hold; high pressure gasoline spraying under the truck onto a hot exhaust pipe could cause a sudden fire with no warning.
If you have a leak in this area, replace the fuel sender with a new one.