Get the right socket on a stiff breaker bar. Pull on the breaker bar until all the slack is out of the assembly.
Hit the breaker bar sharply with a dead blow hammer. It may take a few hits but the bolt should start turning. The inertia/friction of the shortblock assembly will resist turning enough to allow the bolt to loosen.

This is the manual version of an impact wrench.

I just did it on my last 318.

R.