52 Hudson Hornet fuel tank filler trunk floor seal

Your second picture shows the heads of three push through pins.  On the bottom of the floor where these pins protrude are clips that hold them in place.  Attached to these pins are three metal plates. These plates are meant to hold a piece of rubber cut to fit around the filler neck.  Remove the pins cut a piece of tire inner tube rubber to seal the opening, make holes in that rubber to accept the pins using the plates to hold the rubber in place.  In my rendition I used a tube of roofing asphalt to seal below and above.  My repair is water and dust proof after 25 years,   Good luck.