Cut off wheels are the best. Place the new floor pans in over the old floor and draw a line on the outline. I found that trimming from the new floor pans where they don’t exactly fit flush to the existing floor made for a better fit. Cut out the floor about a half inch below the line you traced out on the transmission tunnel and on the outside edges grind the spot welds. This will help everything come out cleaner. Here’s a pic of my removed floor:
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