Just a minor correction to the excellent data provided by 05GTO:
In 1964 and 1965, the GTO was available as an option on the LeMans - not the Tempest. LeMans VIN prefixes (and therefore GTO prefixes) were 222 in 1964 (LeMans was model series "2", where Tempest was model series "0") and 237 in 1965. Thus, it's easy to spot a Tempest that has been converted into a fake GTO, but a well-done LeMans fake can be tough to see.
Additional data on the 1972 engines: The "X" code was a 455 H.O., and the "T" code was a 400 car.