Be sure the sender is grounded for the gauge. Factory used to use a "jumper clip" that clipped onto both the metal ends---the sender end and the fuel line end. The PROBLEM with this is that the fuel line isn't 'really' grounded, either, for certain.
It's a metal grounding strap.
One end clips onto the sender, and the other end of it clips onto the fuel line.
The rubber fuel line goes in-between it. This grounds the sender and fuel line together so your gas gauge works.
Do a search on ebey for mopar fuel ground strap.
It will make sense when you see it.
Problem is, I don't think that corresponds with my sending unit/fuel line. My fuel line is above the sending unit.. and it's not really a straight path to the sending unit. Also, there is a second metal tube coming out of the sending unit (for emission control..?)
All you need is a rubber fuel line between the two steel lines to connect them. Make sure you use fuel line not just a rubber hose. Install the ground strap. The other line can have a rubber cap on it as long as your tank is vented.