My '70 R/T also rusted out in the same spot. Right thru on both sides too! I purchased used frame rails off flea bay, and am presently in the middle of removing the offending rail, along with the torsion bar mount, rear frame rails.... it goes on.
Anyway, I did phone a real nice guy at rust technicians when first deciding what way to go. Can't remember his name, but he started the company, and his personal Charger is what lead him down this road. When I asked the same thing, he pointed out that these cars have been left outside to rot up 'till we decide to rebuild them, and once the frame caps are installed, the car tends to lead a pampered life.
Living in a garage, and being taken care of, means you should expect rust to reoccur at a much slower rate than before, and the car is 30 plus years old now... In fact his Charger has no new rust and he did it over 8 years ago, as I remember. So armed with that info, I simply decided quality primer sprayed into the open top of the rear rails, and torsion bar mount (leave the area that will be welded clean to ensure a quality weld) after sand blasting them clean, would be much better than what had been used before (zippo) and if it lasts only 40 years...oh well!
The front rail is harder to get inside of, and my bodyman tells me that they spray wax with a wand into the holes of open frame rails to stop the oxidization process when they can't actually see the area to be treated. I
can get into the rails as the area under the foot wells are open, and only box in when they reah the engine compartment. Only usefull info if the rail is completely removed from the car, or if you replace the floor pans and blast then, before welding in new ones. Thus I will do the same with the front rails. You
can get in thru the new access hole you have created, so a quality primer following a good sand blast should give you tons more protection than most concours restorations have...
