I've got a garret core on the crown, and it is vastly superior to virtually anything else I've seen so far. The temp drop is awesome.
Sticking with your garret core will be worth the results.
And yep-3mm will be fine.
Sean
PS-Bremen Welding near Parafield airport does awesome aluminium welding-Probaly the best in SA-and great prices. Ask for Dave, and tell him Sean with the Crown sent you.