400 headers @&^#

Only idea I have.. from working on rusty Cat equipment.. get the nuts and bolts hot then immediately sick the wrench on them. If you can work some common candle wax into the threads it actually helps some times.. it really gets in there when the heat melts it. Worked on the track adjuster bolt on my old dozer which hadn't been turned in who knows how long. Keep the fire extinguisher nearby and best of luck.. you'll learn new swear words undoubtedly.