Bucket Seat Swap - Help!!!

My 67 factory bucket seat car only has the stiffener plates welded
to the floor on the inboard holes closest to driveshaft tunnel. They are
welded on the inside of car not under floor pan.
The outboard holes closest to the rocker panels have a brace under
the floor already so no metal needs to be added there.
They used a rubber cup type plug to seal up the bench seat holes.