Buy the LISLE TOOL 22100# tool lock the flywheel and the socket is 36MM 12 point
X2 but if you can't get your hands on one or you're a cheap ass like me, put it in gear and chock the wheels really good.
EDIT: Make sure your socket is a 3/4 drive as well! You will never get it to bust loose with anything 1/2" drive. I broke two 1/2" breaker bars. (A thorsen and a snap-on)
If you plan to twin turbo or a huge single might want to think about tiging the center of the water pump shaft.I wasnt sure which side so i had both sides tig'ed on mine and i did another 1 for back up just in case.
I used to do it the hard way like this then i bought the tool but i like the free pry bar holderI have also heard of guys removing the starter and pulling a socket on the converter bolt and letting it turn till it locks against the rear housing.