I wouldn't cut anything. Be sure your floor is pulled down and tight. I created a wedging device to pull the floor down snug to the frame rails. When I was flat and tight, my floor fit almost perfectly.

I would recommend screwing it into the frame rails when you know it's in the right spot and where you will plug weld later. pulling it in with the screws or clecko's will shorten the floor overall . Again, mine when perfectly positioned flat along the entire frame rail both sides was pretty darn perfect of a fit at the seam of the trunk area and the tab of the frame rail.