It also helps to stitch weld the seam starting the weld on the thicker subframe connector and then carry the weld puddle up to the floor sheet metal.
Another thing that I learned by the time I installed my second set is to grind and shape the connector as close as you can get it to the floor and then use a bottle jack to keep it held up in place with most of the car's weight car remaining sitting on its tires. I used a big rubber mallet to close the gaps between the subframe connector and the floor sheet metal even more once the whole thing was spot welded in place.
