The issue here is you do not want any holes in that fuel
anywhere! They WILL leak. First comes diesel odor then eventually diesel.
If you must plumb things in this way. I would cut the fill line
, install a metal T-fitting, and plumb the return line
into the T. I would do this as low as possible in the line
-- or better still attach the "T" directly to the tank inlet
This needs to be low in the line becasue almost all diesel istallations will specify that the return line run down from the engine to the tank. The sytem is not designed for any back pressure as a result of pumping "up." Clearly that can happen intermittently as a boat piches or rolls though.
Also be aware the return fuel
stream is pretty significant and can generate foaming which could climb back up the fuel
fill line depending on its diameter, and temperatures.