Hi, I'm new around here. I'm from Europe and I have the diesel version of the Grand Vitara (3 door).
About the plates, 6mm in aluminium or 2 mm steel is not enough to support the weight. I weighed mine at 810 kgs up front, anf 700 rear. In the shape I wanted, in 4mm steel I would get 13mm deformation if I held it up at the center with all the weight. In 8 mm aluminium 6,23mm deformation. It is very unlikely to have all the weight at one point (that point being the center of the plate), so I went for 4mm steel. In this case, the deformation would be 13,05 mm, whichwould be too much, but as I said before it is very unlikely to find the exact conditions for so much deformation.
Hope this helps!
sergi