1967 273 stuck water pump bolt

toolmanmike said
Late to the party. The steel bolt is probably siezed in the aluminum water pump housing. You might be able to "buzz" the bolt head with a air hammer to loosten up the corosion. You might have to remove the radiator but you probably will have it out anyway. This air hammer tool may help. 3/8" Shake'n Break Pneumatic

Exactly this ^^^^^^^. No one ever thinks about the bolt shaft seizing up in the timing cover, but it's very common. You can try some PBlaster and tap the head of the bolt with a hammer.