If all else fails, pulling the heads is about the only option. But I would exhaust all other methods first. There is a very good chance that soaking it for a week or so would dissolve the rust or carbon build up.
If this doesn't work, I have freed quite a few engines by pushing down on a piston. I use heavy pullers that I use in my business, but a lot of parts houses will loan or rent pullers for one time use. With 17 tons, something is going to give. I have never had a piston to go to pieces, but if it does, you had nothing to lose anyway. If you can't get away with honing the bores and a set of rings, I would find a different engine. If you are gonna pull the engine, I would have a running engine on hand to replace it. They were expensive to rebuild in their day, and they have become very expensive to rebuild now. And after you're done, you would have an engine that would be almost impossible to sell.
