Originally I would have made my car's floor from 3mm ally, but decided on 1.6mm steel plate space welded to the chassis and sealed with SikaFlex all round the seams.
This gave me a very strong chassis (and floor) without the big number of holes drilled into the chassis bottom for the pop rivets that may cause rust problems later. Btw, pop rivets have no structural benefits.
The difference of weight from 3mm ally to 1.6mm steel was only 3.7kg...
However, for the rear bulkhead and tunnel 1.2mm aluminum was used.