My fuel guage will work but when it gets to half a tank it will drop to empty with in a few minutes but still has gas in it.
The sensor end of the gas gauge is a rheostat. What occurs is the part of the rheostat goes "dead", or no longer reads as it should, which is why it drops to dead empty. To fix this, you need to put in a new tank sensor, which is usually collocated with the fuel pump in the tank.
Odds are Paulster2 is correct. Unfortunately, it seems like you need to drop the tank to replace the sending unit. Make sure a replacement is available before going that far, though. I suspect the original Ford part is long discontinued and aftermarket parts may be special order (not on the shelves at your local parts store). If you're really lucky, the rheostat is varnished up (as opposed to worn out) and you might be able to clear it up with multiple doses of some fuel system cleaner (techron), especially if you let it sit a couple of days with the cleaner in the tank.
If you can get at the wiring connector without dropping the tank, disconnect it. The gauge should read empty. If you ground the yellow/white stripe wire, it should swing to full. This will confirm whether your gauge is 100% before dropping the tank.
If you're going to drop the tank, you might consider replacing the pump and strainer while you're at it, especially if you don't know how old the pump is.