Locked up Slant 6

Try filling the cylinders with white vinegar. Also, don't try to free it by only spinning it in one direction.
Pull the starter out, and with a big screwdriver, work the ring gear teeth back and forth a tiny bit at a time. Be patient and persistent and 95% of the time you can get them unstuck.
Yes, you're also probably going to have to rebuild it, but at least you can start with a running engine.