Installing torsion box

[hemi43]

I wouldn't trim it. See if you can place a floor jack under the tortion box to raise it a bit. I would bolt the end cap on first. Those oval hole are there so that you can MIG through them to weld to the inner rocker. Either way will work, but will be more factory looking from the underside if you don't trim it .

Edit; The torsion box should be last thing that gets welded in. I would fit and weld the outer rocker panel in place first.