New fuel sending unit help.

That's for a fuel return line. If not using it just plug it.
Take a short piece of rubber fuel line. Clamp it to that fitting. Shove a bolt in the other end and clamp it on the bolt. Or other similar item that fits snugly into the fuel line and caps the end.

As a side note what is the condition of your original sending unit? Is it repairable? That sending unit won't read remotely accurate on the fuel gauge. I know they make these little computer boxes to make it kind of/sort of accurate. I've had nothing but headaches with aftermarket sending units.