Floorboard Crack

I have a 1990 LX Mustang and several years ago I had Kenny Brown subframes and jacking rails installed and I don't know if these have anything to do with my problem or not. A crack has opened up in the driver side floorboard. It is about 5 inches long and runs from front to back. Has anyone else ever had this happen? Does anyone know what caused this problem and what will fix it? I wonder if Kenny Brown matrix braces would solve this problem? Any help would be greatly appreciated.
Thanks,
Bob

The floor board crack was a real big issue with the early fox body mustangs. It is caused by all the weight of the seats, plus the driver. I usually have that welded, then I install subframes and a cross brace to correct the issue below the car. On the inside I used 1/8th thick, by 12 inch long by 3 inch wide steel plates that I weld to the chassis, after form fitting it. the car seat is then supported by that whole piece.