You can cut one piece out at a time..
First I would scrub and clean everything out wire brush wise and make everything really clean get all the other trash out of the car as well... Then I would make clear markings with a marker exactly what needs to be cut out all the way up to good material.
Cut out one section at a time again all the way to good material. Do not cut out little pieces.. you want to cut out one whole section like the driver's floorboard or the passenger floorboard for the rear floorboard on either side. If you take out one whole piece as in a quarter of the floorboard at a time you can reconstruct that piece out of flat material. get it welded back in and it's old position using the piece you took out as a template to trim your patch piece... Even if you have to weld small patch pieces down before you take it out to connect that separated area... remember all this is going to get covered by carpet so just making it super close will be fine. Also it will be 10 times stronger when it's one whole piece... Save your money for a good carpet kit so you can cover it all and never see it again... Should be well under $200...
