Weld them up the same way as the cracks for the seat mount. Would also suggest stitch welding the seams where the torque box contacts the floor pan and subframes. Helps with strength and stiffens things up. Do this on both the uppers and lowers. Look for pulled spot welds and address them as well. Welding the uppers will require pulling the back seat and having fire watch. If you are by yourself you can soak a heavy towel and cover the area you are welding in the car.
Wanted to add to my comments above. You will need to remove the UCA's and
LCA's from the body mounts so you can also weld up the seams that are on the inside of the torque boxes and upper mounts. Also, cannot stress this enough, clean the areas that you are going to weld. The cleaner they are the better the weld quality will be. Start with degreasing everything then move to a wire wheel (you can get 1/4" shank wire brushes for your cordless drill) and get all the surface rust, paint, etc. off the weld area. After you weld them up wire brush everything again, wipe down with some brake cleaner, allow dry time, use an etching primer, and then either paint or use undercoating.
I just removed the rear end and drive shaft so my buddy had a lot of room to weld things up. If you do not want to change transmission fluid at this time you will need a tail housing plug to put in the trans when you pull the drive shaft. You will also need to plan on plugging the brake lines. I also ended up cleaning up the rear end (power washed, degreased, wire wheeled, and painted) while it was out of the car.
Is it a lot of work? Yes, but if you do all of this your car will thank you and your wallet will later down the road.