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Hagerty Employee

How to identify Pontiac engine blocks | Hagerty Media

Pontiac blocks are interesting power plants that are often misunderstood and improperly identified by sellers and buyers alike. The blocks of all Pontiac V-8s from the 287 to the 455 are the same size and very close to the same weight, which can make it more difficult to identify the 11 different engine displacements produced from 1955-79.
New Driver

This is all great information. My dilemma is my 1965 GTO cowl tag is dated 12B-1, meaning it was built the second week of December, and was built in Pontiac. It has a 428 in it, but I want to find an original Date code correct 389 WT block to build to make the car as close to numbers matching as possible. People are telling me that my car was built in late 64 for model year 65. What would be the appropriate block information to look for? Should I be looking for the Block unit number or the last digits of the VIN #?
Thank you in advance.