What I did was I under cut the floor slightly and then just perimeter welded it all the way around using .035 wire because I was to lazy to change the spool.
I started on one outside edge and jumped from left, right and the middle until one side was done, then I went to the opposite side inside tunnel and did the same, then went to the other outside edge and repeated the steps and finished off with the inside opposite tunnel.
I then ground the weld flat to the bottom surface making sure not to grind the corner edges, I only did this where panels would be later fitted.
Unfortunately I didn't take any good photos, but do have a portion under the fuel tank that shows the what i am talking about before finishing and painting, it is sort of an example of the whole floor.
It seems like less work than drilling a lot of holes and spot welding them all, just my opinion only.
Also if the floor is welded all the way around I think it will provide better protection from the road elements as well because there are no exposed edges or seams to the inside.
Maybe try taking some scrap pieces of sheet used on the floor and clamp them about 1/16 or so in from the edge of a scrap piece of 1 inch tubing, clamp it in a vice and try welding, start the weld on the corner of the tube and blend the weld into the edge of the sheet floor and back down into the corner of the tube in a circular motion maybe going a 1/4 inch at a time.
By doing that you wont weld long enough for any burn through, once you get the feel of doing that, you should be able to change to a stitch type weld if you choose to and not have problems with burning through.
Most people have the settings correct, just need to add a little rythem
Al