[tbirdtbird]

Yikes, outside with no head!!! that hurts

not sure about the steel plates. I'd vote for hardwood. Not sure I would trust the ring lands after what you are gonna put them thru

be advised the cyl walls where the water jacket is will be quite thin; maybe only 1/8", be careful.

glad you have done this before.

Given all the new info, I'd borrow or buy a large needle scaler, and use it along the circumference of the piston once a day as well as using the wood block and a hammer daily. A needle scaler will set up a lot of vibration, which is key to loosening without damage.
I had to work on a '59 Caddy with a frozen dizzy on the rear of the engine. Could not really get back there. Could not be timed unless we figured out how to loosen it. Grabbed the needle scaler and set it on there for about 5 min a day, along with penetrant, for about a week, it finally broke free without damage.
You will chew the pistons up with a scaler but as mentioned I am not sure I would trust them anyway

Mitch might have a way to use his induction heating equipment here......

If the block is out of the car, crank out, and such, putting kero in there and lighting it off might help it along as mentioned above. Doug has done it and we have done that too. The trick here would be to not get it so hot as to melt the babbitt. I'd prolly do one cyl at a time with kero and then the needle scaler, and avoid the heat of doing the entire block. If you were careful you could control the heat well. This method won't harm the cyl walls since they will heat up slowly, and they were made to heat up from combustion, anyway. A dry ice approach might be too rapid a temp change and crack something; it is -109° after all

If you used kero 1 cyl at a time, you would not know when that cyl was free, but you could chip away at it this way.

If the head was off, would water have gotten into the water jacket and cracked something when it froze? You may have a can of worms here.....