[Gav]

The only reason that the later engines run 4 O2 sensors it to monitor the cats.
2 will work fine.
However having said that, since you are using an engine that was designed to run in Australia with 4 sensors or EGR? (LS3) you will probably find that your engineer will stipulate that it will need these to pass irrespective to whether they make a difference to the emmissions or not.