A HQ turret will be a problem for originality, even with the vent flutes welded up. But if it's for your own personal driver then I guess it doesn't matter. The difference is in the air ventilation system and you need the full roof lining bows and finishing teeth strip near the upper tailgate (HQ's never had these). GMH won't tell you much. You need to contact Holden Historical Services (nothing to do with GMH) www.holdenhistoricalservices.com.au and obtain a report. The records may show the correct engine number for the car (might not also). Original sales papers and car history are best and add the value more than any other. Anyway get onto Ben at HHS.