Long wire Brussels bore brush, cut handle off and attach to a drill, could hot tank it, I would likely just wash it out and spray some penetrant - it’s sealed in the axle tube, long as it’s clean when you reseal it’s moot
Any chance you’ve attempted loosening the other bolts off before installing them by hand or possibly double checking the mounting hardware/mounts for proper configuration?
Usually a bracket that attaches to the manifold I thought, any wrecker really, but aftermarket heads may have to fabricate one, I’d watch that thread,.