I started out using some Aircraft Paint Remover to remove the paint. I've never used this stuff before, but let me tell you, it REALLY works well.
Basically, you spread on a layer of this stuff over the surface that you want to remove paint from. Be very careful to not get this anywhere that you don't want the paint to come off.
It is apparently very caustic, so wear the proper protective equipment (gloves, goggles, long sleeves, etc.). Also make sure to do this in a well ventilated area, as the fumes are relatively strong.
The product causes the paint to blister up after about 5 min. After you feel like it's blistered up enough, use a plastic scraper of some kind to scrape off the old paint/primer that is now mush. Please dispose of the remnants properly....a clean planet is a happy planet. Rinse the work area down with water to neutralize the paint stripper, then dry it off and admire your handiwork.
If you are not going to use POR-15 to your bare metal floor pans, you will want to primer the area to prevent flash rusting. Since POR-15 is made to go over rusted surfaces, I'm not too concerned with the flash rust issue, so I'm leaving them as is until I get the whole thing done and ready to be coated with the POR-15.
This is what the passenger floor panel looked like after using the Aircraft Paint Remover (post clean up)