Flip - '69 Firebird Build

[CarQz17]

First time doing a build thread so bear with me. I bought a 69 firebird a few months back and spend that time getting it driveable. I rewired it with MAD electical's new system, installed a new edelbrock carb, manifold and air cleaner and a few other odds and ends. Ran great but I hated the way it rode and it sounded like it was going to shake apart. Thus I bought a full DSE suspension for it and started digging in the other day. This is what I found and the work that ensued. Whoever had the car before me did almost as much work to cover up the damage than what it would have been to do it right. They were good, I give them that. Keep in mind I am 22 and doing all of this in my 1 car garage in my apartment. Here it is...
















I did some more work tonight and found some other big holes. I will post them in the morning when I get a chance to take some photos. Looks like I'll be doing under seat pans, trunk pan. inner rear wheel wells, frame rails, and probably the tail panel. Basically the whole rear half of the car. Will I need to brace the car to keep it true as long as the quarters stay on?