I'm hoping that Bob will chime in with at least some helpful hints, maybe he(they) have a replacement sending panel that will have both a fitting for a return line, and still be able to have a working fuel level gauge. Trying my best to not have to drill tap a new orifice to the tank, as that would require draining and removal of the tank, Also hoping that Bob and Peter do not mind me posting these two pictures to this thread for clarity.
Having never had to remove or inspect a 289 FIA tank, it appears from the attached fuel filler diagram that there is a spacer of significant height (yes, I know it is on a lower area of the tank) to potentially allow space for a return line set up of some kind.
The tank itself is a 19 gallon, well baffled unit, so the current fuel feed is perfect.
Bob, Bob, Bob, anybody, anybody
Bill S.