I am visualizing a multi-ton hydraulic floor jack with some type of arrangement with a chain to strap the block to the jack,,,or some other mechanism to make it immovable,,,and a very stout metal (metal pin or some such) held against the pistons from top or bottom. And the other end against the jack pedestal.

Something like placing block onto a hydraulic press and pressing them out. Have no idea how to manipulate and manage a block of several hundred pounds onto a press,,,,perhaps possible using a cherry picker,,,or perhaps the heavy duty jack idea may do it. Under extreme pressure and a few well placed slams with a big hammer may work things loose.

As I said, just visualizing.

Don't give up.

Likely you have checked out all the heavy duty liquid wrench type products. There are a wealth of them. I have often read of auto transmission fluid mixed with acetone that some swear by.