seized slant six

After you use the lubricant of choice on top
of the pistons, I usually try turning the engine
over by the flywheel with a flywheel wrench.
I get more leverage there.
I move it back and forth small increments until
it frees up so I don't ruin the rings or score the cylinders.
After it frees up I run it through a couple of cycles by hand
then with the starter.
I usually mix Marvel in with the fresh fuel to help lube and
clean things up.
Hopefully its not stuck too hard and the rings are not
seized in the ring lands.
Good Luck!