Where and who is doing the Euro 6 testing?
Curious how complicated the test is?
Note, you are in Austria not Australia, for anyone else that miss reads things.
I would check what fuel you need to use and investigate if you can use a fuel of your choosing during the test. I have been doing some initial research into what effects different additives and fuel recipes have on emissions. If you believe what is published, it may be the case that adding things like Ethanol to the mix will reduce the emissions. You may be able to use this as a tuning tool.
Here in Australia - not Austria, we only have an E10 blend, which is 91RON with up to 10% Ethanol added, bringing it up to around 94RON. I have read that other places around the world may have a 95RON or 98RON mix with 10% to 15% Ethanol added. This would put it up around the 100RON mark, which would reduce its nock sensitivity. If you get detonation / nock this tends to send the NOX levels up. The combustion of Ethanol also results in a different % of exhaust gas particulate outputs.
I don't know enough about what is being tested and what the values are for Euro 6.
You really need to know what the output of a standard LS3 will be with the standard tune and configuration.
Is it one of the LS3's that have the cylinder deactivation?
Will you be fitting an auxiliary exhaust smog pump? This just pumps extra clean air into the exhaust which dilutes the % values of the emissions, not changes the emissions that much. It may help in allowing the catalectic converters to work at an optimal level.
Not sure if Euro 6 accounts for additional clean air being pumped into the exhaust. That was certainly something that many manufacturers were doing in the 90s and 2000s to get through testing. I suspect Euro 6 may have closed that loophole though so that's probably not going to help.
Ryan