I had a closer look under my 70 deuce and noticed the frame has rust through near the firewall where it parallels the rockers. Then I noticed a previous owner welded 1/4 inch steel plates in front of the arch over the rear axle. I felt around and noticed a ton of debris in the frame where the rust through occurred. Now I have a plan to clean out, weld, and seal the inside of the frame rails to prevent further corrosion. Any of you guys with these big car frames might want to check and clean out these areas!
The 1/4" plates that you are describing might be factory. The full-size "tow pack" cars received additional bracing in the form of plates on the outside frame rails above the axle. I'll try to get a picture of mine in the morning. Derby guys look for this depending on the class they run.