'68 Pontiac History

[september10]

I've had the car for 4 years and have reached the point in my restoration project whether to replace the engine with a new crate version, or rebuild the current. The current engine is 428hp out of, I was told, a Bonneville. Originally the vehicle had a 350hp out of the Factory. I don't know who put in the 428, but was told Dealerships used to perform this conversion as a "special" package before being sold (Royal Pontiac?). If this is the case, I'd prefer to completely rebuild this engine as this car could have special value. Other than ordering the Car's History from Pontiac Historic Servces (ie. Automotive Services, Inc.) what advice can you offer to determine where my car was originally sold and how determine the origin of this 428?
Thanks a Lot!