I did a complete floor replacement on the passenger side twice (don't ask), and have yet to do the driver's side. I reinforced the door openings and took the body off the car.
The first time I used a spot weld drill bit that has a flat face to drill out some of the welds where I was concerned about retaining the metal behind where I was drilling, for everything else I use a drill bit just a little larger then the weld. You can get a spot weld bit at good parts houses, Eastwood, or Harbor Freight.
The second time, again with the body off the frame and up on saw horses, I cut the floor out with a cut off wheel, then ground the welds off. It takes some care, but goes as fast if not faster (or was it just practice). I did use the spot weld bit in places I could not get the grinder into.
If you have an air compressor that can keep up with air tools, being smaller, they're the best in my opinion.
Oh, I did the floor first, leaving the sill and rocker in place to help support the body, which gets real flexible with the floor out.