if you are going to bolt the x member straight to the floor, id make it 100mm wide, and along the flat part of the floor and up the side of the tunnel, on the top side of the floor, then stitch weld it in
So would 100mm x 100mm x 5mm be reasonable sized plate to spread the load?
depends. if its in the middle of a flat bit of floor, possibly not. if the plate overlaps a crossmember or goes up the side of the tunnel, then it might be fine. hard to tell without seeing it