The replacement of Chevrolet floor pans instead of just sheet metal is the difference between a restored classic car and just a repaired old car. Restoration of a car, especially a classic Chevrolet, must be done to original specifications. This is the only way its real value can be obtained on the open market.
The replacement of floor pans on old Chevrolet models is very common and necessary. Not only is the bottom of the floor pans exposed to the salts and moisture from the road, but the upper portions are exposed to what the passengers of the car bring it with them when they are sitting in the seats. This again includes moisture and salts.
From the interior of the car, carpeting generally covers the Chevrolet floor pans in most models except way back before the 1950's were only floor mats were used. Because of this the only way a person would know if replacements floor pans were used to repair the holes in the floor or sheet metal are riveted into place would be to look under the car. Sheet metal will be seen clearly from there and its use will take away from your car's originality and value of being considered a classic car.
The time required to place simple sheet metal instead of the original equipment floor pans on your Chevrolet is another deterrent for using it. Plain sheet metal will have to be cut, in some spots welded and the rivet holes will have to be sealed to prevent moisture from entering the passenger compartment. This is a labor intensive process. The original equipment parts fit right in and require just a fraction of the time while making the car more original and increasing its value.