I butt welded in my partial floor pan but also used a reinforcing strip on the bottom side. I used a 1.25 wide by 3/16 thk steel strip which is available at Home Depot, Lowes, Sears. I fitted the panel in place, then tacked the strip to the bottom of the existing floor sot hat half the width extended into the repair area. This gave the rear floor panel I was installing some support in an area where not existed before. With the panel in place I tack welded it around the perimeter and then to the support strip. I could then weld up the seam ont he inside of the cab. As extra insurance I used seam sealer on all the floor welds and seams.
