Here is how you get 3 advantages when placing the floor sheet metal below the top side of the floor tubes (and NOT at bottom):
1. You keep the welds around sheets far from dirt and water. An additional replaceable protective composite panel is bolted at the bottom.
2. Increase stiffness. A chassis floor structure with sheets between tubes welded close to the upper side is stronger than a floor with whole sheet on top.
3. Lower the weight (since you remove the portions of sheets where the floor tubes go through).
You place the sound-deading material inside the 5 mm tall holes above the metal sheets.
This is CAD calculation of the volume of a whole sheet on top of the floor compared to the volume of 14 separate sheets laying 5 mm below the top of floor tubes (when using sheet with thickness of 2 mm):
(whole sheet) Volume = 1722.14437 cubic centimeters
(separate sheets) Volume = 1394.77973 cubic centimeters for 14 sheets
With reparate sheets between floor tubes, you save 327,36464 cubic centimeters of metal. To give you an idea for the amount of saved weight, a 40x40 mm tube with length of 1100 mm has volume of 326.84594 cubic centimeters. If you measure the weight of this tube, you can easily see how many kg you save.