The floor pan is only 18-19 gauge metal so welding thicker plate to that does gain anything. A thicker plate on the underside should spread the load and increase strength. Do you have pictures of the pan/seat area?
Here's the driver side of a '69 Camaro which has a U-channel welded into the saddle from rocker to tunnel. Seat mounts bolt in to nut plates welded on the underside of the top of the channel.
AMD may have channels for the Nova???