Unless you have a "K" code you will not have an engine number on the block. But yiu will have a casting number cast into the block that when de coded you can tell when and wher the block was cast. You will have to remove the starter motor to view the casting number.
Underneath the engine, between the starter and oil pan will be the engine casting number and date code. Its usually hard to read from underneath the car, in the tight space between the starter/oil pan, and with the oil/grease buildup thats probably covering it. However, it is possible to read if you can clean it off some and have a good flashlight. Write down the numbers and google "mustang casting number decoder", it will explain everything about your casting number/date codes
If you're looking at the front of the engine, just to the right of the distributor and below the driver side head will be a small flat piece sticking off the block with numbers/letter stamped on it. This is the engine assembly date code... the exact date the engine was actually assembled
Also, in the lifter valley under the intake manifold there will be 289/302 stamped on the block itself. If you take the valve cover off there will also be 289/302 stamped onto the top of the cylinder head as well. The heads could have been swapped out however.
There is no way to 'prove' matching numbers on a non-K engine since it had no such numbers. Every part on the engine/car has its own date code. If those date codes are not all reasonable for when the car was built (i.e. BEFORE the assembly date by a few days/weeks/months depending upon the part) then you can prove the engine is not made of original parts. You can never actually prove that they ARE original parts or that they are THE original parts.
To re-iterrate what IVY66 said, unlike Mopars and Chevys, there is no "numbers matching" Mustang with a specific designated engine. Basically if the engine code in the VIN matches the engine displacement it will in most cases be probable that it is the original motor or at least fathomable that it was built for that car.
That's my biggest pet peeve is seeing an advertised "numbers matching" Mustang. Usually I know they have no idea what they are selling which could be good or bad for the buyer.
Yupp theres no actual "numbers" just a casting number and date code... as Ivy said, if the date codes on the engine code for a few weeks before the date on the VIN, then in all likelihood it is the original engine because it would be a difficult task to find an engine with the correct date code to match your vin number
if the date code isn't within a few weeks of that on your vin then its deffinitely not the original motor. good luck
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