[joe_padavano]

There's a very easy way to tell if the engine is original to the car. There is a VIN-derivative stamped on the block. There is a pad located on the drivers side of the block at the front, below and just forward of the number one exhaust port. The pad is usually covered with crud and will be partly obscured by the power steering bracket. The VIN-derivative is stamped on this pad.

The VIN-derivative is a nine character number. The first character will always be "3" for Oldsmobile Division. The second will be the year of manufacture - a "4" for 1974. The third character is usually a letter and designates the manufacturing plant (M = Lansing, Z = Freemont, CA, etc.). This letter must match the seventh character of the VIN. The last six digits of the VIN-derivative match the last six of the VIN from the car that the motor was originally installed in. All numbers must match (hence the term "numbers matching") for this to be the original engine.