I've always thought the original "definition" of a muscle car was always 2 door, V8, 4spd, 4barrel and the engine used was pulled from the next size car up and not available elsewhere in the same series. This is why the HK GTS327 is the only Aussie car that fits the bill. The identical spec 327 engine is shared with the Impala and Parisienne sold in parallel with the HK but not available elsewhere in HK. The HT and HG miss out because there was no bigger car with a 350 fitted and also the 350 engine is shared elsewhere in HT and HG on the GTS350 auto. If you exclude the requirement to be a V8, then the LC XU-1 fits as well. And you could also argue the A9X hatchback does too, although the existance of the 4 door A9X squashes it.
All an irrelevant discussion in Australia anyway as the 3 x highest performance 1970's Aussie production cars: late HG GTS350, Phase 2 Falcon (some will say Phase 3) and E49 Charger really deserve their place as the pinnacle of performance from their respective manufacturers.