I think I would R & R the transition piece that goes over the differential first. The leading edge of the trunk floor overlaps that piece. Try to remove the
crossmember last as that ties in the ends of the frame rails. If 1/4's are removed, It would probably be easier to weld and install the drop offs to the
trunk floor as a unit. I have several of the transition floors over the diff. I think that 63-76 pieces are essentially the same dimensions with some small
differences mostly in the pinion snubber area. I also think the diagonal braces are the same for all those years.