The frame doesn't LOOK too bad in the photos that you posted, but that could change, once its sandblasted/cleaned up. I think you might want to consider using that Malibu as a parts/donor vehicle, and start looking around for a car with a good body and frame on it, as a basis for your build.
FWIW, I ended up in a similar situation last year. I bought a 1981 Cutlass wagon, that only had 76,000 original kilometers on it (about 45,000 miles), and on my initial inspection, the car looked good, so I bought it. After I got it home and started tearing into it, I found a lot of rust issues, similar to your Malibu, to the point where the body shell was basically no good, and would have required replacement. I would have had to sink WAYYY more money than the car was worth, into fixing it up, so I found another vehicle, that was in better condition, body & frame wise, to fix up, and used the wagon as a donor for parts.
What I've found over the years, is that you're much further ahead to find a car with good body/paint/frame on it, and do mechanical repairs/upgrades, than having to pour money into a rustbucket.