My wife and I set 20 utility poles, 12 to 15 feet, in 3-foot holes, using a small tractor, chain and an ancient design for lifting poles. First we set a greased 2x6 in the far side of the hole for the butt end of the pole to slide down(instead of tearing up that side of the hole). Our lifting device was a pair of 12-ft 2x6 boards, bolted together in a distorted X shape: a cross brace in the middle made it look like an giant A, but another 6" to a foot of 2x6 extended above the top of the A. To lift, we set the feet of the A a-straddle the hole, held the top of the A about 3 feet above the pole, fixed the chain around the pole near the top, then through the X at the top of the A, then over the hole to the tractor. As the tractor pulled horizontally, the A-frame converted the horizontal motion into a vertical lift on the top of the pole. The butt slid into place against the 2x6 waiting in the hole, and pretty quick the pole was high enough to slide into the hole. The chain does not slide through the crotch of the A-frame; rather the A-frame rises ahead of the pole and ends up leaning against the chain about a 30° angle towards the tractor. I suppose we moved the tractor a bit more than the length of the pole; I don't know if a come-along has that long a cable. A big hazard was mis-estimating the layout, in which case the pole tried to sweep around about 2 feet off the ground looking for somebody's leg to break. When I got on the tractor, the wife's job was to stand at the top end of the pole to steady it as it started up, but never inside the possible radius of a sweeping pole. These were 10-12" creosoted utility poles and quite heavy; your 6x6 stock is surely a lot lighter.
Good luck, and let us know how your job progresses!