The engineers at GM-Holden changed some things to suit local suppliers when the design came to them from Korea, and the engineers at GM-Chevrolet changed some things to suit their local requirements when the design came to them from Korea.
No-one changed the same things, or in the same way.
Some changes are really small but critical - the headlights, for example, are about two millimetres different in size, and they use a different globe fitting.
If you can get access to the global database that they selected the bits from, you could work out what is the same, but the only body kits that will fit a JH are those from Walkinshaw/Irmscher, if you can find one, or maybe something from China, if you are lucky.
I've had issues doing the exact opposite, turning my chevy into a holden. The best route would be to source a set of headlights and a complete front bumper with all the trim installed. Shipping to Australia will cost a fortune though. I made due by cutting the center bar out of mine and putting a holden grille on.
As you can see, there is about an inch gap below the grille because the headlights are slightly different size and shape, and the grille is about a half inch too narrow on each side.
Aside from the front bumper, everything else should be direct fit. rear bumper, horn pad, trunk badges, engine covers on certain models etc.