The fuel sender is just a old fashion re-o-stat.They can be adjusted by removing the tin cover, but most likely the resistance coil needs cleaned.Yep these girls are showing their age. The after market replacements should be checked before installing and matched to the gauge.Connect a jumper wire at the sender wire at the tank,a 12gauge wire about 8 feet long to reach the driver compartment.Connect the wire to the sender terminal with the unit in your hand and with another jumper wire ground the sender to the body.There you can check it with the key on,move the float arm to half tank and see what the gauge reads, check at full and at empty. You can adjust the re-o-stat sweep(if needed) before installing it in the tank. Just a thought.
