I know you have already done over these things, but my dads 73 chevy had the same kind problems. Turned out to be a combination of a loose bumper bracket, worn out shock mount, and tailgate. The shock mount bolt that goes into the frame was worn out and loose, I welded it and that reduced most of the noise.
I didnt see this mentioned anywhere, but have you checked your drum brakes to make sure everything is in its place? Other than that...Im have no idea without being there to help.
- Shane
