The hidden reinforcement that would make a difference would require opening up the car and based on your rust free description, I doubt if that's a good approach in this case. How deep do you want to go is the question.
As far as the dimension, are you measuring from the hub centers? If yes, do you have all of the lower rear control mount shims out, are the upper rear arms adjustable or extended, and are the front upper control arms caster modified? I do not measure the wheel base from the centers. Might I suggest measuring the platform from the suspension mount points side to side and see if they match. Use the control are tabs/holes front to rear. If they match, do a cross measure left rear to right front and see if the numbers are the same. If yes, don't worry, the car is fine and will align correctly. DO NOT USE THE FACTORY ALIGNMENT SPECS! They will take out the inside tread on the rear tires.
The S car is suppose to have the same center to center wheel dimension but is indeed longer. I know this because we had to modify the platform fixture to bolt on the body.
Here is a shot of a simple fix that helps keep the taillight corners from cracking. It's a bar welded in to hold the rear valance from flexing when you're driving the car. Picture is a bit muffed---it's the muffler opening area on the rear valance.