Check your firewall on the inside. Pull back the carpet and look for rust stains. Also check your #2 mount area. These areas tell all.
Not likely.
The firewall is fiberglass, so you won't see any rust on that, unless the birdcage is so severly rusted that it's dripping stalactites like the Titanic. The only metal parts on the firewall are the doublers around major components, and they don't tend to rust.
The #2 mount area doesn't collect much moisture, and is one of the last to rust, unless the birdcage above looks like the Titanic... In which case you should put your '80 on the trash heap and get another car - it's simply not worth fixing.
Corvette frames, when they rust out, will rust out in the rear kickup area. This is because the rear frame blockoff plate drains tend to get clogged by debris, and water will gather at the rear blockoffs. If there is no rust at the rear kickup, the frame is likely solid. If there is rust through the rear kickup, it warrants further examination, but even this is no evidence that there is rust in other places. Surface rust is no indication of a "rust problem."
Here is a frame from a Vette with no firewall rust issues, no birdcage rust at all, and the #2 mount looked like it was fresh from the factory. This is the rear kickup on a car with "no rust":
You need to do a thorough examination to determine what is simply "surface corrosion" and what is a "rust problem." Surface corrosion is handled with a wire wheel, elbow grease, and a bit of work. "Rust problems" are handled by removing the body, cutting sections out of the frame, welding, and sandblasting, on a car worth the effort. If your '80 needs that effort, dump it and get another car.
Lars