The inner sill is drilled, and plug-welded to the floors. You can use a spot-welder here, but it better be INDUSTRIAL strength – I’m talking something that is 6 feet high, water cooled, etc. The usual tong-style air cooled units will flex too much if the tongs will reach around the inner sill, and you don’t get the clamping pressure you need for a safe weld. Remember, along the recessed footwell, it’s THREE layers now – not two. We don’t mess around here, we plug-weld it.