My understanding is that ALL high performance engines were stamped with the complete VIN, even in '67. You may want to check on the driver's side bank at the rear of the block for a flat pad which MAY have a partial VIN stamped on it. Also possible is a partial VIN stamp on the back of the driver's side cylinder head. The partial VIN stamping was required for the '68 model year so it may or may not be present. However, if you have a '68 or later partial VIN you'll know the engine was not original. If you're lucky, maybe you'll find an engine assembly stamp somewhere, but I doubt it.
6F09A 63A 8 26 09D 71 1 5