IMHO, reinforcing the floor where the seat bolts to the floor is not necessary. During most collisions, the seat sees very little loading - it's the seat belt that takes the force. I suppose a massive rear end collision could tear the seats from the floor, but that would have to be some collision.
In fact, look at how the seatbelts are mounted to the floor - just threaded through a flat bit of steel maybe 3/4" X 1" or so, presumably to spread the load a bit more than a standard hex nut. It's tacked to the underside of the floor board, but I always figured it was for the convenience of not needing a second person to hold the nut while installing the seatbelts on the assembly line.