Take dimensional before measurements. Slice the seam sealer with a good razor blade, cut the welds at the back and unbolt everything. I would highly recommend installing the bedsides with the new floor on the frame so that you can ensure good body lines. Ever seen a truck with the bed wider than the cab? Don't assume that it will just bolt together without a lot of trial fitment and modifications. If its an aftermarket floor you may need to elongate the holes and recoat them with some rust protection as they generally never line up. Get a tube of seam sealer, some lacquer thinner and an acid brush. Don't weld the corners until the floor is bolted down and the gate fitment is perfect. Do this on a level surface with the tire pressures equal, jounce the suspension and eyeball it from every angle.