I am plugging along in my restoration of the 1969 Fiat Spider and today I removed the carpet and some of the tar and other covering. I saw there was some rust at the meeting of the firewall and floorboard. There are also some smaller holes closer to the door sill. Overall the floorboard seems really solid in all of the other sections. I think I may just cut the bad areas out and make some metal patches to weld in. I could also go the route of a new floorboard from Wolf Steel, but I am not sure how high up it goes on the firewall.

If anyone has advice on the better way to go, please let me know. Picture of the work follow.

First shot with carpet removed:


Rear seat area is very clean:


Front passenger footwell shows signs of trapped moisture:


Driver side does not look bad at all:


So after a wire wheel cleaning, you can see the holes:

The corner of the floorboard, sill and firewall is structural, so I agree that area should be welded. Smaller holes elsewhere can be patched like this: http://dcfiats.org/tech/POR-15-2.pdf
OTOH, if you know how to weld, there's nothing wrong with using metal everywhere. It's particularly easy on the floors, where appearance or a flush fit is unimportant.

You might want to get under your car and poke around with a ice pick to check for thin metal. Be sure to check the metal under the seat supports, the rear cross member that supports the front of the back seat and around the brackets where the lower trailing arms attach. I would also remove all the tar material just to confirm the metal was good then paint it with MasterSeries Silver Primer/Sealer and cover it with a modern sound deadening material (the Stinger Expert Roadkill works great). I became a much better welder than I ever wanted to be when I bought my Fiat Spider! Good luck.

You also might want to check under your wiper cowl just to make sure there is no rotting metal that rainwater gets through. you seem to have a lot of rust in the passenger side.

I already cleaned and treated the area under the cowl with POR-15. It had only surface rust whereas the floor is eaten away in a few places. There will be some patching.