Gas Gauge Sending Unit

My friend, who has a 72 Duster 340 4 speed car has bought at least 4 after market senders. It was all for nothing because they are "junk". We have checked the resistance outside of the tank before installation & all is good. Minutes after installation the readings are all over the map, 5 gallons of fuel reads nothing, 10 gallons of fuel reads 3/4 tank, drive ten miles, fuel level drops to empty. We are now searching for someone to rebuild the original & hopefully it will work correctly.
Your best bet!