Hi Tesgt350,
Both those owners manuals should also have the owner/dealer info filled out with the SAI VIN as the warranty number. This is the only way to prove an original Mustang owners manual came from a GT350, assuming no tampering. I would value them equally because they both have the same pedigree. However, the stamping would add value. I do not recall ever seeing an original manual offered for sale. Vern might have an idea.
Cory