[Three Peaks]

I wouldn't cut the body lip there. Pry back the carpet and drill out the rivets for the aluminum. Then slip the fiberglass body channel between the aluminum and the chassis tube and let the aluminum set on the cockpit side of the body channel and rivet it back in place.
Then, you'll need to make up an aluminum channel to fit over the door jamb and carpet to protect it. It will be much stronger and support that portion of the door jamb and frame chassis this way. It isn't designed to fit over the carpet and aluminum, it is designed to be put in place before the aluminum tub is rivetted in place.