The frame rail is only 3 sided with the TOP of the rail using the Floor to box it in. Once you cut the inner fender out the floor will LIFT off the frame rail. Spot welds will not cure the problem there. Welding it off completely will stiffen it all up like Scott is saying.
I actually welded the floor back to the rail and then GRINDED it back down flush, then I installed the inner fender with the strip of metal, I tacked it in with screws and then scribed from the inside of the trunk along the floor pan on the back side of the INNER well. I then took it apart and cut the INNER well to that line, put it back in tack welded it all then FULLY welded inside the trunk area. I also went back into the wheel well and FULLY welded the from the outside also inside the wheel well. So I have 3 welding passes along the rail.
1) floor to frame rail after inner tub removal then grind flat
2) Inside trunk
3) Outside along rail again in the wheel well itself
