When we evaluate rusted metal, we use an awl or an ice pick. We are not gentle about it, we are looking for weak metal.
If your metal is sound, but just pitted- I would look around for a shop that can do spray welding. Parts that you dont want welded can be masked with a special gel. Its a lot of careful work to knock down the spray welded sections to make things look nice, but you will never know it was pitted.
If the metal is weak, I would replace those sections.
You could use DOM tubing. Anyone with a good size lathe should be able to turn you up correctly sized pieces in the 6-8 inch long range. Its a PIA but it can be done.
Its certainly not a "deal breaker" on a frame, its just another adventure in the restoration process.
Lee