I had to torsionally rigidity test mine to get it engineered for registration here in Australia. I have metal right through mine (metal hood liner, metal console, metal parcel shelves, full metal dash etc). We welded it al together. but the engineer also wanted to "strips" across the car in the floor (note car has new rear frame, new AMR front clip , and welded in subframe connectors). Even with all that he still wanted the two strips. They were 6mm steel (thick) by about 2" wide. Contoured to the floor plan and welded in. Well torsion test. It's stiff than an Aston Martin. Takes over 12,000 Nm to get it to twist