I wouldn't waste my money on the repair section, I did both my sills, first one was using the two proper repair panels then second one was with a piece of retangle section of the same dimensions as the sill.
Plan is to cut out the old sill from the floor down, clean up the underside of the floor, offer the box section up and jack it in place, then seam weld the whole thing in.
It means less disturbance to the B pillar (as if you do the full repair section you need to cut everything out and the door pillar is left hanging (with a piece of metal temp welded to it as a brace somewhere).
You can also cut out any corroded areas on the floor and weld them all up.
This method if done well, is a stronger repair, easier, cheaper and quicker.