Don't know about GA, but in CA, you have the choice of registering a "Specialty Constructed Vehicle" by the body year, or by the engine year. This is only for first time registration.
Since the GT40 was never a production car (except for the few MkIII's), you would say it's a 1965 and be required to have the smog equipment that was in effect for the year.
In CA, we are limited to 500 per year. So far, we have about 450 still available for this year.
Bill