Floor pan rust!

So I was just curious and stripped away some of the carpet on my dart, and what I was kinda expecting to find some rust because my car is unrestored. But my main question is how do you know when the entire floor pan has to be replaced? Because there are no holes in the floor that I could see, but it was kinda flakey in some spots, was still fairly solid though. I know replacing a floor pan isn't too bad, and if I have to replace the whole thing can I just strip the interior out, cut it out, replace it and weld and seal it and be done? Or do I have to brace the car since it's a unibody? I hate rust! It kinda made me sad to see it since the rest of the car is in pretty good shape.