Paul,
I had the EXACT same issue on the driver side, and I also a hole punched in the bottom on the other side where there's a circular plate from the original manufacture... looks like the jack plates crushed slightly different spots.

My advice is similar to an earlier reply, with one difference - metal epoxy.

I did the following with the crack like yours:
1. bought a 2-part metal epoxy (PC-7)
2. molded it into a pencil thick cord
3. jammed the cord well into the crack
4. pounded the floorboard down as described by earlier responder
5. let it dry for a day
6. sprayed it liberally with an aerosol can of undercoating

On the side with the hole, I:

1. cut away the original undercoating around the hole and ground it smooth with a sander
2. pushed out the disc
3. scored the disc and metal outside around the hole to help the epoxy and undercoating adhere
4. drilled a small hole in the center of the disc
5. made a cord of epoxy as above and placed between the disc and the bent-up hole (I cleaned it up with alcohol to help the glue stick)
6. drilled a hole through a spare long piece of thick metal rod
7. inserted a sheet metal screw through the rod and into the hole in the disc.
8. When I tightened the screw, it pulled the disc downward (the rod was a brace across the good sheet metal around the hole), squashing the cord of expoxy into the bent underbody metal of the car - effectively filling the spaces created by the errant jack
9. let it dry for a day
10. removed the screw and rod, and used a little more PC-7 as a putty to fill the screw hole and to pretty it up.
11. let dry overnight and sprayed undercoating on liberally.

It's been a hard, snowy few months since, and it looks like new.