fuel sending unit and fuel gauge trouble

The link to aftermarket tank kit purchased shows a 70-10 ohms sender which is close enough to Chrysler OEM gauge.
Use a volt meter to check at the sender. You're looking for a pulse voltage 2.x, 3.x, averaging about 6 volts. If there's no voltage there you could check at the rear harness connector behind the left kick panel but... a fault there is extremely rare.
The temp gauge operates on the same pulse voltage. If it doesn't work either, the limiter that produces the pulse voltage is suspect. If the temp gauge does work just fine, The fuel gauge may be loosing connection where it attaches to the printed circuit board with pal nuts. This is a somewhat common fault.
Sure there are other possibilities, like broken connector pins on the circuit boards. Most of these inst' panels have been removed before for one reason or another and that's how the connector pins get broken. So... If your tests determine the panel must come out, be careful. Consider installing a new limiter, new light bulbs, every service practical, while its out.