Just looking at your pictures, I would say the cab mounts and floor areas around them need to be addressed. The left rear looks like there isn't much holding there.
Not sure if you can do the work without removing the front fenders or not. if you need to replace the rockers then I'd say you would. I believe originally there was a lip that turned up where the floor and rockers meet??? I guess you might just patch the rockers and leave the front fenders on....
John
