If you just have to use something to stop the piston then at least do it so that you pose the least risk to the engine. Turn the engine over (head down) and turn the crank so the exhaust port is open. Pour in about a teaspoon of fuel or oil. You can't compress a liquid.
Yes, if you have to torque to the nut to extremes you might bend a rod. But if you have to crank on it that hard the piston would already be damaged from ANY other type piston stop.
When installing a flywheel first always make sure the mating parts are grease free.
Ed M.