Fuel level gauge problems

by "sending unit" do you mean the fuel pump housing or the float assembly, because they are in seperate locations on mustangs.

if you changed them bolth you may want to try another guage cluster,

do the rest of the factory guages work?

The sending unit is a variable resistor that reads approximately 25 ohms for empty. If the gauge is always reading empty then either the gauge is stuck or it is always seeing 25 ohms. You could try connecting different resistors to the wiring to raise the level if the gauge does not respond to this then the gauge is bad.