You are correct that ZB is a production code for a 1974 350. As far as the vehicle history, the only way to get that is by tracking down the previous owners. Most states won't release the owner history on a vehicle (if they have it) due to privacy concerns, so you will have to try and work back from the guy who sold it to you. Based on the VIN of the car, Wayne Roberts of the GS Historical Society might be able to provide so information based on the factory documentation, but what happened after the car left the factory will be from previous owners only. This link has Wayne's contact number:
http://www.stage1registry.com/stage1info.html
Buick did have a replacement engine program to replace failed engines, see this post:
http://www.teambuick.com/forums/show...0189#post90189
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