I just got my truck so I'm not sure if/when I'll do this (since I don't know what I'd put back there for one). But if I do it, I'll likely span the front and rear bolts with a solid piece of steel...I'm no mechanical/structural engineer (but I know one that used to work for Ford so I might ask), but I'm going to make the assumption (like a previous poster) that the gap, granted only 1/2in or so high would cause some type of problem. If not in every day driving (although a 300lb man sitting back there may cause issue), I'm guessing in an accident wouldn't be the same. Or perhaps the seat back hook is less strong (not strong enough) lower down on the hook (the open end).
So my question after that is - is the floor flat between the bolts, so a flat piece of steel would work?