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Must be a WA thing.308 can be legal if running cats and EFI. Mate had a couple dramas registering the motor to his car but after explaining couldn't find a spare 304 block and the machinist supplied a 308 rebuilt motor with EFI heads and all the injection they passed it.
His was engineered , maybe because he did an emission test is why it worked?Must be a WA thing.
In SA and most of the eastern states if you tried to do that the engine number would get flagged right at the start of the process and your application would get put in the bin.
In SA the engine number has to be from that variant of car or newer aka VN-VS VU prefix block, and old mates car being an S3 would need a roller cam prefixed block technically as the emissions we're stricter in the S3, but can't be older even if you retrofit emissions equipment etc.