sorry for the crude drawing
left is book design right is rework all in 3mm
in the book design the blue areas are not fixed the red areas are either welded or bolted. when you press on the brake pedal it pulls up the floor and pushed out the middle on the 1mm to 3mm joint. this creates a rocking motion
in the right picture the 1mm and 3mm plate has been replaced by a full 3mm plate. this plate has been welded directly to the chassis rails rather than the floor and unbraced 1mm plate. it has also been welded in the blue areas removing the rocking motion created by the poor fixing points in the book design.
another solution would be to keep the book design with the bolt fixings but also weld the blue areas to remove the rocking motion. but to be honest if you havent made the plates already its faster just to fabricate one 3mm plate.
i am keeping the book design as i have already got the precut plates but the blue areas will be welded to stop the rocking motion.