Well he cut them out after I left, but I think he just used a cutoff wheel on a die grinder after marking where he wanted to cut and the whole thing dropped out. You can see where it's welded on the underside and on the door sill. I don't think there was any other cutting, the 2x6 just slides right into place.
He has some 2x4's cut to the right length to hold them in place while he tack welds it in a few spots.
we had to pull the carpeting back, and there's a few places where the wiring sits on the floor, so we pulled that up. The undercoating is a pain to weld through and where the sheetmetal welds to the top of the slider is tricky, but other than that, it looked pretty easy.
The pinch seam is completely cut out and re-welded on both sides of it to the slider.