* As someone else stated, the motor number is on the driver's side of the block, located not far from the back. It's also useful to know that the number is stamped (not cast) on a milled-flat, rectangular area which is also close to the head gasket interface (top of block).
* Sometimes there's enough grime or rust that the numbers are barely visible, if at all. Some solvent on a rag followed up with a little wire brushing should make them clear enough to read. A lighted magnifier help.
* You're looking for a number in this format: 617.912-12-XXXXXX. (I'm assuming it's naturally aspirated and automatic.)

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Quality Enterprises
Mercedes Svc. & Restoration
Sheridan, IN
317.769.3536