That truck came with a 180 degree T-stat from the factory. I don't care what Autozone's website says.
Try a different radiator. You can "flush" a radiator all you want at home. It can still be clogged. Either that one might need to be rodded out, or you need a new radiator.
You set initial timing with the vacuum advance disconnected and the port on the carb capped off.
That fan shroud is ill fitting too.
Gary