The easiest way to stop the crank turning in the car is to make a tool out of 50mm x 5mm flat bar steel. Length around 750mm, but this depends a bit on if you are working alone or with a helper. If alone it needs to wedge against a frame rail or the ground to stop rotation. Drill 2 holes in one end of it with the correct spacing for 2 of the balancer to pulley holes. Use the pulley as a guide. And don't use 2 opposite holes otherwise you can't access the balancer bolt. Bolt the tool to the balancer and use a breaker-bar to undo the bolt. This tool will also allow you to torque the balancer bolt correctly on installation. Don't forget to use some sealer on the keyway to avoid an oil leak. If the t/c and seal was replaced 3 months ago don't even think of replacing the seal. They press in anyway.