For that you would be looking at several thousand dollars in repairs. Just for an example if it needs a complete floor those are $3-400 a complete trunk floor is around $300 all the braces for the body mounts $200 and those are without shipping and install labor added on. For the frame it may be cheaper to swap out the entire rolling chassis depending on how common G bodys are in your area.
This isn't to turn you away from working on your car but to give you an idea of how expensive it can be. Take it one step at a time and you will be able to get your car back in shape for another 30+ years.