Have you had the fuel pump and sender unit out of the petrol tank? If so take it out again and have a look to see its working correctly and not fouling on anything. Also check the plug and wiring looks intact and has no bad contacts that would affect the return signal voltage.
Does the tank empty light still come on when the needle is still in the position it did before the problems? If not and its way under, then it may be just the needle has moved on the spindle. emove the clocks from the dash, take the needle off (use a fork to pry it off) and replace itwhere it should be. To do this you need to refit the clocks with no glass on it, either have a full tank or the empty light on and replace the needle where it should be.