For $900 you bought yourself a project. Rust problems are bad to deal with. It's a shame such a nice car wound up in that kind of shape. If it gets too expensive to deal with, at least you have a nice pile of parts to create an SS clone with. That may well be cheaper than doing all the work it will take to get to all the rust that's in this car.
I haven't seen such a rusty Chev since a friend of mine bought a 1973 Monte Carlo over 20 years ago. Like yours, it looked good at a distance but once you saw what was going on, it was flat out scary!
Rick