The front sills on my '48 were completely shot, coming all to pieces. The rear sills which are either original or repops are still in good condition. The price of new sills was cause for hesitation so I decided to try to come up with alternatives for the front sills. Here is the material I ended up using:
The vendor markets this rubber mat as running board material, I never could find it as a generic product. A 3' length of the 18" width is just right for splitting to make two sills. I used a pry bar to pop the old sills loose, the rubber is vulcanized to a metal base. Barbed pins on the bottom of the base snap into rubber grommets in the body sill. The old base could be used as a pattern for the new sills but I used poster board to customize the pattern. After the pattern was fabricated it was used to cut galvanized metal sheet to use as a new base and this was riveted to the body. Double sided flooring adhesive tape secured the new rubber sills to the galvanized sheet. The pattern of the new sills is similar enough to the old sills to be more or less compatible for my driver car.
The adhesive tape is really aggressive and I don't anticipate it coming loose, but if it does contact cement can be used.