Welding that 1/4" plate to the tunnel floor is where most of the strength comes in because it is being supported by the floor. The thickness of the material does very little and is mostly extra weight. It is also difficult to get good weld penetration when you weld dramatically different thicknesses together. In my opinion, you should just use the upper tunnel floor itself and box in a 1/2 - 5/8" tall channel with a more reasonable 1/8" plate. That would likely be stronger without all the unnecessary weight.
