Roger (Roadrunnerman) has some good advice. No rolling Jig or rotisserie, get 4 jack stands. The bigger the better, usually equates to how tall you can raise them. The higher up, the more room you'll have underneath to navigate around. Get the car level. Use a level (I prefer the digital with the included water bubble). Looks like your rear package tray would be a good place to put the level for your side to side leveling. Setting it on the flat area of the rocker inside the door opening is a good place to level front to back. Use rubber shims between the jack stand head and the bottom of the car to tweak the balancing. I just use chunks of mudflap. I don't use metal to shim being it would be more prone to slipping off.
If your frame rails are still attached to the rear crossmember, that's a good thing. At least they're still tied in parallel to each other. If the rear crossmember is still tied into the rear tail/fill panel and trunk pan, that's even better. Odds are it's still straight back there, unless the car was damaged/twisted in the past. Next, use a plumb bob and mark from key points on the frame rails (in the same spots) to the floor. Mark the floor with a permanent marker where the plumb bob centers out. Mark the area on the rail where you held the string. Now, measure between the mark on the frame rail and the mark on the floor. Take pictures, mark the floor, document it, make a drawing..what ever suites your fancy, but never the less, don't lose that data. The more reference material and measurements you take the easier it will be to re-install and re-install correctly. Also plumb bob mark/measure other critical areas like the center cross support, shock crossmember, shackle mounts, fill panel ect...ect... the more the better. One you're done with that, examine your results and see where your at as far as being straight. Start with all of that first, then worry about removing what's left of the trunk pan, extension panels and what ever else may need replacing/repair
Good luck
ops: Next I want to remove the dash and see what shape the firewall is in. I do like the idea that the original color can be seen in the window frame. It's Metallic Silver. Every car I own right now is silver lol.
