This is the second "Gantry" I've built on my trailer. The first was just under 6' high and people kept hitting their head on it. This one is close to ten feet high. I can hang a snatch block anywhere along the beam to get the log where I want it. This one is made with Pressure treated 4X6's. I unhook the truck from the trailer and run a 17,000 pound bull line through the pulley. Tie the end around the log about 3 feet back on the log. That way if you pull up just a few inches from touching the pulley, when you back up and let the log down, it slides forward on the trailer. I do put chocks under the trailer tires, but, the force of the truck pulling down on the gantry keeps the trailer from moving. When I pull the chocks out, they are loose.
If you don't have a dump trailer, make some short chain or rope chokers, and leave them on the back end of the log. Back up to a tree and tie off to the choker and pull out from under it.
The full load was loaded in less than a half hour, and it was our first day playing with the new gantry,