1967 273 stuck water pump bolt

Does the bolt go into the block or just into the cover/housing?? If it threads into the cover you can use a propane torch (oxy/acy would be faster) to heat the aluminum around the bolt while using a wet rag to keep the bolt as cool as possible. Once the aluminum is hot it should expand allowing you to turn the bolt?? You have to work fast so that the bolt doesn't soak up much heat so it doesn't expand with the aluminum. Aluminum expands faster than steel so you have one advantage!!

Treblig