How to fasten

Look behind that crank pulley if there two, opposing surfaces, that you can apply a large but thin wrench to lock the shaft from turning. Also, if you can not "lock" the crank from turning, you can take advantage of the weight of this assembly, and tighten that bolt by hitting your wrench with a hammer. Giving it a sudden impact like an impact gun would do. As last resort, remove the starter and wedge something in the flywheel teeth to prevent the engine from rotating. The flywheel is large enough diameter to give you a really strong mechanical leverage over that bolt. If it's a fine thread, be careful not to over torque or strip that bolt/hole.