Yes and No. In the California DMV system it is called "Special Construction." The car must pass the emmissions requirements in effect for the year the cars was constructed if it is driven on the highway. So, in most cases, the only way to "build" a new Firebird (or Camaro) from the ground up will mean to use an LS2 out of a GTO with the entire ECM. If you have a title and the VIN plate to an older car, then you can get away with what I would call a "resurrection."

However, you might have to imprint (i.e. restamp) the hidden VIN numbers properly in order to pass the DMV's inspection.