Structural Integrity Of New Metal On Old Body

Not a difference except some of the new panels are a bit heavier gauge sheet metal . Replacing outer body sheet metal should be no different on an older car than taking a new car in to have a quarter replaced that has been wrecked . I can see replacing panels that have numbers as sticky area ,however you should be able to replace any damaged panel .

These guys actually go a little over board with these bodies .Shelby and other racers raced these cars for years without problems ,and they were not gentle with them .
Rust proofing would be the best thing that you could do .Prime inside and out ,there were a lot of places these old cars never even saw primmer, like up inside the quarters , and look how long they lasted any way .
Seal them well with seam sealer ,if water cant get in it doesn't rust .Cowl areas are the worst ,it is hard to prime or even paint inside .An old garden sprayer with a wand and spray up in side the cowl ,rockers and any other hard to primmer spot will stop a lot .
Seal the windows well ,the biggest problem with rotted floors is leaky windows .
New door seals ,trunk seals and any other rubber seals will help stop a lot of rusting problems.
Keep all the drainage holes clear ,in some cases enlarge the drainage holes such as the bottoms of the doors .
The doors them selves need to be primed well inside .
Most of these cars will never see the use a regular driver will see ever again .My last 65 fast back saw less than 2,000 miles a year . I had it for 10 years before i finally sold it .

One thing that really needs attention is the 65s and 66s ,they do not have torque boxes in the front .The only thing holding the frame rails in place ,or the whole front end for that matter ,is the fire wall and the floor supports. Torque boxes attach the frame rails to the rockers and the floor supports .I bought a 65 fast Back that had been in a front end collision and it shoved the frame rails,and the motor , into the inside of the car taking the fire wall with them and ripping the floor clear out all the way to the seat boxes. Torque boxes will help stop this ..They a fairly simple to add .
67s only have one torque box and adding the other side is a very good idea. 68 up have torque boxes on both sides .