I cut out a section of the driver's side floor and replaced it with a patch panel I made.


I made a 'hammerform' of the shape I wanted to duplicate out of some scrap wood.


A bolt through a hole in the center of the 'plug' area held the parts together. Then a big hammer put some shape into the panel.


A couple of bends in a sheet metal brake gave me the final shape.


I traced the new panel's size & position on the Alfa's floor pan. I cut inside the marks I traced and then slowly nibbled away until the fit was satisfactory.
I 'stitched-welded' the panel into place with a MIG welder. Stitch welding means I placed a number of small, widely spaced welds, let them cool and then went back and placed more small welds between the previous welds. And so on and so on until the panel was welded all around its perimeter. This minimizes distortion.
Next I ground down the weld beads. Finally, I will clean it all thoroughly and apply some paint to stave off further rust. (All this will, of course, be under the carpets likely never to be seen...)


Here is clue to the source of water that soaked the carpets and promoted the corrosion.

Peeking up under the dash you can see a rubber hose. This is a drain for the cowl vent (the grill in front of the windshield).


Next is a view looking up behind the dashboard. Looks like I'll have to add a couple of drain hoses to my parts list...