I just did a quick search, and it looks like the stock sending unit should read 0 ohms when empty and 90 ohms when full. It sounds like someone put a junkyard pump/sending unit in the tank with different ohm readings. I don't think that you can just swap wires, but I am not an electrical guru either. Different manufacturers use different ohm readings on some sending units, i.e. 0 Full-90 Empty, 120 Empty-30 Full etc.
edit:What kik said, try switching wires first, it might work?