I agree that it sounds like you will not lose any geometry with replacing that inside section, but again, can't say for sure unless you post good clear well lighted pics of the rust area. gumbi is right about: you never know how far the rust goes until cutting and peeking in. This is why slow and careful is the deal. Take a little at a time. Don't cut too deep in case there are other supports inside. It is probably a DIY job without supports needed, but can't see it yet.
Sort of weird that only the inside is rusted and not the outside eyebrow. Condensation? Leak holes somewhere? in front drip rail?
Gumbi took off way way way more than you are indicating. I would not have the guts to do what he did. I would buy another cab.
