If your using a post hole gear box, find a hydraulic pump that is a speed of about 540 rpm, and then the gallons that your tractor can provide, and a full auger may not be the best many use a short winged bit, if your not careful one could stick a full auger
if all else fails one could do or use a bailer to get the spoils out, as in a cable rig, one would have to pull the auger out,
those twisty post hole diggers, I know a guy who drilled a shallow well with one of them I think he had some type of ladder to reduce the times he had to break down the pipe in the process of removing the dirt.
so one could use the auger and only dig about 3 foot, and then pull the unit up and dump the dirt, much like pulling a well,
if one made a boring bucket, basically a pipe with a auger head that will hold the cuttings to be removed,