I just worked on a 216 and a 235. Both were were frozen.
I would take off the head. You will probably do this anyways. I ended up doing it. And move the pistons around. You will find the problem or which ones are stuck. If they are.
On one of the engines I used a mixture of dextrol and brake fluid in the cylinders. I read that the brake fluid would soften the varnish (old gas) and the dextrol (oil) would do lubrication. Then let it sit for a few days. You can use this or Mystery Miracle Oil. I tried that. I am not sure what worked the best. I tried Kroil, too.
As someone said "pop" the top of the pistons with a wooden dowel and hammer. I did not use a big hammer but kept popping the pistons around the outsides. Mine moved after a period of time. This time may be soon or much later.
I also found that the wrist pins in the pistons and rods were sometimes frozen. I had the engines on stands so I would flip the engines over and loosen them. Again, I found out that in the end the engines were completely torn down so that was not a problem.
Oh, be careful. Your soaking fluids will go around the pistons and make a mess if you don't have anything under it.
Best of luck. I am sure that somebody with better knowledge and expertise will give you advice.
These are just my thoughts. I am a novice at best.
