Yeah Tom;
In my shell off renovation I found that my wheel wells (on my 63) were the main reason that the floor had to be replaced. My answer was to replace the bottom 6" of the tub with new galvanized metal. step 2 was (is) to seal both sides inside and out with automotive spray on undercoating. I beleive that this should seal up everything very tightly.
Remember you guys with the old vintage rigs that the factory undercoating does dry and become brittle so it cracks and falls off. A good preventive maint tip would be to inspect these and respray them on a regular basis.
