Rear quarters definitely need some work!
Have you though about cutting off the rear quarters at the same height as the bottom of the tailgate? And just capping it off with sheet metal?
Two big upsides to that idea are simplicity and you can run a high clearance bumper similar to the one I built, that wraps around. I’ve never worked on a 60 so I dont know if there are setbacks or difficulties with doing this, but just a thought!