Well, 2 weeks and $500 later it looks as if my floorpans are solid again. After peeling away endless yards of rotted soundproofing and rusted metal (in some cases, they were identical in appearance) and grinding/ wire brushing away as much rust as possible, I started having metal welded in...

The first job was the rear portion of the passenger floor, which was clearly the worst. This was done professionally and they did everything, including sealing and weatherproofing, it was $350.

The 2nd part was the front portion of the passenger floor and the front portion of the driver's floor. These were done by a welder who just agreed to MIG in some new metal for me if I took care of sealing, etc. This was only $75.00.

Using the opportunity I tore out all of the carpet and plastic trim and deep-cleaned everything before replacing.

Starting with new metal, I covered well with Q-pads, 1 square foot self-adhesive tiles purchased from the local auto store. They are expensive but work great. Then I reinstalled the carpet and seats.

Everything smells fresh and new, which is nice. The new metal was welded in overtop the old so I've used minimal expanding Great Stuff and then spray undercoating to seal it very well.

Sorry for the long post but thought you might be interested in the process.