Cutting out the top of the crossmember like that is just a mess and doesn't leave enough metal to do anything. Then welding a section of strap below it is another messy chore that; again, doesn't really afford much stiffening. Just slice out the top completely; everything above the bolt holes by slicing thru the floor from the top; and right above the bolt holes from underneath. Then weld in some strap from the top; flush with the floor; and tacked to it. Finish up with a wider strap over that; welded all the way around it. Then you have a stiff floor. With about a 1/4" bulge if you use 1/8" metal. Barely raises the carpet. Drill some holes in the last strap before installation; so you can weld both straps together.
