2) Most PTO shafts I have seen on Post Hole diggers , ( and just about everything else ) are 2 piece , With 1 sliding over or inside the other to allow for movement as the implement is raised or lowered , especially a hole digger . My neighbor rented one for a weekend and we used our TC30 to dig holes for a arena and the shaft would get longer or shorter as you raised or dug deeper into the hole . Not to mention that when you connect the shaft to the PTO of tractor , you need to have movement .
So if 66" is the total over-all length , I would think that a collapsed length in the vicinity of 54" to 56" would be enough to cover the distance and supply enough overlap to remain stable .