fuel sending unit and fuel gauge trouble

I can tell you this much.
Maybe the fuel gauge isn't working. With the ignition key on, briefly touch the fuel sending unit wire (the wire that plugs into the sending unit on the gas tank) to a good ground. Have a helper tell you if the gas gauge needle moves. it should. If not, try holding the wire to the ground a little longer. Also, with a volt meter, you should have a reading of about 5 volts at the fuel sending unit wire. If there is no current in the wire, and the needle did not move when the wire was grounded, you can next check for current at the back of the gas gauge. If there IS current going through the gas gauge, and none going out, the gauge is likely bad. If there is NO current flowing through the gas gauge, the IVR (instrument voltage regulator is likely bad. On a 69 Barracuda, the IVR is built into the gas gauge, and the gas gauge will have 3 terminals on the back: One terminal for 12 volts into the IVR, one terminal for 5 volts that will flow through the gas gauge (and that 5 volts also flows to one of the two terminals on the back of the oil and temp gauges), and one terminal for the current flowing back to the fuel sending unit wire. If your gas gauge only has two terminals on it (not sure when that happened), the IVR is not in the gas gauge, and is easier to replace. Also, if the IVR is bad, the oil and temp gauges will not work, but if they are working, your IVR is likely OK.
Your gas, oil and temp gauges all have a bi-metal filament in them similar to what is found in your home's thermostat. When current flows through that filament, it heats up and moves, causing the needle to move. The more current that flows through the filament, the more movement you get. The current flows through the gauge and through a wire all the way back to the fuel sending unit. First understand that the fuel tank MUST be well grounded! The fuel sending unit contains a rheostat that allows the current to flow from the gauge to the car's ground. With a full tank of gas, the sending unit's float goes all the way up, and the rheostat lets a lot of current flow through to the ground. Since you now have a lot of current flowing through the fuel sending unit (and, consequently, through the fuel gauge) you get a lot of movement in the needle, and it tells you your tank is full. If, however, there is no gas in the tank, the float is all the way down, the rheostat is not allowing any current flow through to the ground, there is no current flowing through the fuel gauge, and the needle will not move. So, the bottom line is that for your gauge to work with the fuel cell's sending unit, the sending unit must operate like your original (regulating the amount of current that flows through to the ground), and the resistance in the fuel cell's rheostat must be similar to what a 70 Mopar sending unit would be, and the fuel cell must be properly grounded. I am not saying you cant get your gauge to work with a fuel cell, but if all you are doing is hooking the fuel tank sending unit wire up to the fuel cell, this is likely why it isn't working.