Need help with install

Are you sure your thermostat isn't sliding down in the housing since it's mounted side ways? I mean is there enough clamping force to keep the thermostat in place when it's tightened down and not slipping after it's installed? Also make sure there is no air in the system. Looks like with your configuration that the housing would be an easy spot for air to get trapped. Your really not that far off of 180. Center on the gauge is 180 and from your pic your almost there. Have you taken it out for a drive? Good luck finding the culprit.