You need:
-1/2" breaker bar and 3/4" socket for the crank snout dampener bolt. Anything less will fail.
-Around 10-15 feet of nylon rope will a diameter around 1/4" or slightly less. It should be soft yet dense enough to not flatten out when squeezed.
-Iron pipe from the nearest lowes/home depot that will fit over the handle of the breaker bar. Longer is better, so 5' just for the breaker bar is nice. Steel stock works too as long as it fits over the handle, and has a wall thickness greater than 3/16". Buy pipe, its cheap.
Get another pipe the same length, for a scissor effect
The head needs to be on the block, torqued at least some and not just beyond finger tight.
Remove a spark plug from the head and feed most of the rope into the chamber. The goal is to have the piston compress the rope to the head around half way up.
Figure out the best position for the breaker bar to lend the most leverage while still leaving room for swing. You will need that pipe over it, and they will flex some.
Position the other pipe to counter the breaker bar effort or the motor will tip way before the bolt budges. I placed the second pipe between the crank dampener pully and the water pump, with the end sticking out beyond the exhaust side of the block.
Hold and keep pipe 2 from going down to the ground while applying force to the breaker bar/pipe with gradually more pressure until that bolt gives. Stay square in the effort and no sudden jerking or bad things will happen.
5 feet is a minimum, more leverage may be necessary.