Fuel sending units

Please clarify, "will not read at the gauge".
A Mopar gauge is meant to receive a signal in a non-linear resistance format. Therefore you must use a matching signal in order for the gauge to read it properly. Attached are the physical differences between the two types.
All Mopar fuel sending units house the resistor up at the mount at the tank end. aftermarket ones are mounted half way down the tube. I hope this clears this up.
You can use an aftermarket sending unit but you have to change the way the stock gauge receives the signal.
First pic is a linear resistor the second is non-linear

Linear Board.jpg

Non-Linear Board.jpg