Seems like a waste of money and a good way to destroy a Pontiac, but to each his own....Given that you don't have many of the original bits, are you sure it was a REAL GTO? Are LHD not an option in Australia? If they are, I think keeping it LHD and sticking with true Pontiac bits would be the way to go IMHO. You can find a period correct engine in the US, have it built, and sent down to you.
1965 GTO Convertible 3 speed tri-power drum brakes originally
400 cu in with tri-power
4 speed Muncie