Assuming in-cab tank, the line off the top is the fuel supply heading towards the carburator. It goes down through a fitting in the cab floor, through a flex hose and connects to the fuel line which on my 57 GMC forward on the inside of the right frame rail to the engine compartmment.
My truck has a new fuel sender fitted, where does the pipe that comes off the top go. it looks like it should have a pipe attached. Is it a vent pipe?
Thanks