1975 (and the first half of 1976) is pre-ADR27a, meaning that the only emissions requirements are to have a charcoal cannister fitted, and under 4.5%(?) CO at idle.
The CO reading can be verified anywhere with a calibrated exhaust gas analyser (not the full EPA closed room test).
The NCOP states that if you have a later motor, then you should meet the emissions standards of the year the motor was made, but this is not actually what the legislation says, and some engineering signatories will be willing to follow the legislation over the NCOP.
For later model vehicles, it is a bit murkier. The existence of the B2xA and B2xxA motors with factory carburettors, mean that most RWC inspectors won’t notice or be upset by a B2xxF or E motor that has been fitted with a constant velocity SU or Stromberg carburettor.
In the context of a 240, the most extreme example is if you fitted a pair of Webers to a 93 model.
The strict letter of the law says that this would need full EPA testing to be legal, but there are issues with actually obtaining that testing in NSW at the moment, and regardless, many engineers will recognise that a properly tuned pair of DCOEs on an enthusiast’s car will probably be cleaner than a typically poorly maintained LH2.4 and may make allowances for you. They will still want to see operational EGRs and evaporative controls functioning, but the mere existence of a carburettor is not the end of the world.
Finally, as these cars age, the number of people in the mainstream automotive industry (including the law enforcement side) who are familiar with them, is rapidly diminishing and the benefit of the doubt is easier to come by.
So… “you can’t retrofit a carby to an injected car” is a gross simplification of a complex topic, but it has always been closer to a false statement than an accurate one.