what mm ali to use?

[deezee]

I've used a 2mm 5251 H22 for the floor (210MPa Tensile Strength). I then used 1mm 1050 H14 (Half Hard) for the tunnel, behind seats, sides (100MPa Tensile Strength).

If your bolting your seats into the floor (as per the book) You want a beefy grade of metal, plus your going to stand on it. I think the book actually recommends using 2mm aluminium for the floor. 1mm to 1.2mm for other panels. If you want to enter you car into the 750mc race series, it needs a steel floor.