Try gear centre in Edmonton Alberta Canada, they may be able to give some guidance where to get them if they don’t have them on the shelf - try rock auto by chance??
Copy and Pasted,
Locate the engine block code. This series of seven to eight digits begins with a letter and is stamped directly on the engine. Look behind the distributor on the passenger side of the block if it is a six- cylinder GM engine.
Look at the first digit, which will be a capital...
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?