Fuel tank sending unit for 1966 Dart?

Unless you have a large V8, you're okay with the 5/16" line. That said, it costs just a little more for the 3/8" output. Most 3/8" senders have a 1/4" return line which needs to be plugged. There is one maker of a 3/8" sender that does not have a return connection. There was a recent post on this about a month ago.

Everyone says to use the original locking ring on the sender to prevent leaks, because it is thicker. Just FYI, the original locking ring thickness is NOT always thicker than the replacements.