The center section of the pan is in good shape. A little work with the wire wheel and it will be as good as new.

The right side is marginally better than the left, but still destroyed. Note the damage to the firewall flange.

All of the rotted steel was cut out for replacement. I wanted to keep the seams from being visible once the car was done so the firewall patch was placed as close to the rear inner fender as possible. The cowl side panel spotwelds at the front door pillar were drilled from the outside of the car as it was much easier then from inside. The spot welds were also performed from the outside. The original cowl side panel was cut from the inside behind the door pillar so the seam would not be visible behind the kick panel when completed. I chose to cut the firewall patch around the original heater mounting hole to avoid potential mounting problems. The remaining steel was returned to shiny metal using a sanding disc, cup brush and wire wheel on the grinder. The replacement cowl pans were fitted over the rotted ends and their locations marked. I left as much original metal as possible and cut the new pans to fit. An extra 1/2" of overlap was left on the new pans. I chose to underlap the new pans (old over new). 3/16" holes were drilled every 1" in the new panel where it lapped the firewall, and cowl side panels for plug welding. The lap joint was tacked and the seam welded to the old pan.

The cowl pan was plug welded to the firewall and cowl side panels. Then the lapped end was welded to the remaining portion of the pan. Spot welds every 1" to fit the pieces tight. Then spot welds between the spot welds. Then again until there was 1/4" or less between spot welds. Then the remainder was welded solid. After both sides were in the welds were ground flat. Then the entire surface was cleaned, prepped and coated with 2 coats of POR-15 (http://www.por-15.com). I used POR-Patch to seal the can to cowl pan joint. They shouldn't leak again any time soon.

View of the right cowl side panel which was rusted through at the floor as well as at the cowl pan.

The right side firewall or what was left of it prior to removal of the floors. Note that nothing is holding the right front frame rail to the rest of the car.