The ps seems solid, unless its thinner than it appears, I don't see a real problem there. Ds, I would cut out a section of the lower rusted piece and find similar guage metal and replace that section. Now if its thinner foward of that are, best thing would be replace the rail... that goes for both sides. The crossmember, I'd replace the full piece for sure.

I had to do various patching on my rails in the challenger and I did alot of poking around to find any thin areas. I also had the floors cut out to replace them which made it easier. How bads the rest of the car, cause usually when they look like that underneathe, the rest needs alot too.