I just looked up the Chevy 6000 series - not very likely the rear end is from a 6000 is it?

Not likely...6000 series would be dual wheel...
Open drive shafts are what we have in our modern vehicles, you can see the shaft turn and the U-joints are exposed. Closed drive shafts are called torque tube rears, the U-joints and drive shaft are inside the tube, you can't see them spinning...looks like a solid pipe running from the transmission to the rear.
Mike B
