ADRs have nothing to do with what you're talking about, they mainly relate to safety items. If a Holden, a Toyota, and a Mazda all have ADR27A, that doesn't automatically mean you can fit a V8 to all of them.
Don't confuse ADRs with the various codes of practice for vehicle modifications. Those codes of practice, such as the NCOP, are what set out what can be modified both with and without an engineering inspection.
For as long as I can remember, the non-engineer limit has been that you can fit any option which was available from the factory for that model vehicle. So you can take a six cylinder LX Torana and put a Holden 308 in it without consulting an engineer, but if you fit a small block 307 Chev you do need an engineer's approval, even though the capacity is less, because a Chev motor was not a factory option for that model.
There was no factory V8 option for the UC Torana, so any V8 fitted, even a Holden V8, and even though it's a direct bolt-in conversion, would need an engineer's certificate.