For these really difficult to separate parts, try Oil of Wintergreen as a solvent to wick through the rust and loosen things up a bit and reduce the size of BFH required.
Back in the day, when I was a young engineer at the power plant, we used this oil as the penetrant to loosen the large nuts and bolts holding the both steam turbine shell halves together, at the 5 year rebuild cycle. This oil loosened the threads enough, so they could be removed with only slugging wrench and small sledge hammer. In the years before using this solvent, they broke off several of these 1-1/2″ bolts with just brute force. The older mechanics swore that oil of wintergreen was the very best penetrant to use, as it would get into places the other fluids wouldn’t touch. Herb