Not sure I'd spend the time removing the old sound/"rust"proofing material. I see some pretty big holes on both floor pans. My driver's floor looked like a twin of yours, and we didn't bother to remove the old coatings; just cut it out and replaced it (along with both sets of seat rails). Passenger side was closely inspected, and found to be fine. THAT needed scraping!
I tried a couple of different methods, but a powered multi-tool with a scraper blade worked best. For large flat areas, I used a spider blade, which fits in a recip saw; but when that blade hits a seam or a transition, it can buck pretty bad. Small, narrow areas got the cold chisel/ hammer routine. Did mine in midwinter Wisconsin, so the dry ice method would NOT have worked...

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However much you do, it's some work, and takes a while. But well worth it once it's out, and you get those first coats of stabilizer and primer on the tub.
Neil