New fuel sending unit help.

That is the larger 3/8 outlet. 69 used a 5/16 outlet and no return.

Plug it like mentioned and know that the connection hose to the your old body line may be to small now and can pose another leakage issue.
Unless you are running a high power motor the 5/16 line is fine and I would return it for the correct one. Don't forget the ground strap!