Do what I do to mount the outboard seatbelt... Drill a 1 1/2" hole in the bottom of the
subfloor
box,  Feed a 1" x (?)" long piece of band iron drilled and tapped, or welded nuts on it... to
use as
the floor mount for the seats under the sheet metal floor.  And then use a 1 1/2" plastic
body-plug
available at most auto-parts stores on the "HELP" rack.... to plug the holes.

Ray