Personally when going to NGK for those I use Iridium plugs like the BR8EIX or BR9EIX they will last longer but they cost more. I would run NGK as those were what Polaris recommends now and have for a long time for those Indy 500s, is it the liberty engine or the silver Fuji Engine? If it is the silver then the Iridium plugs come pre-gapped and you don't mess with those gaps or it will screw up the plugs.