Just saw the post were you had drilled a hole for air to get by.Hey big daddy, throwing this out there. Way back in the late 70's I built my first small block and had a over heating issue, shut it down immediately, didn't want to damage it, plus money was tight. Reached out to one of the old timer mechanics and was told about an air pocket under the T-Stat, and was told to drill a 1/8" hole at the T-Stat plate and reinstall, that it would allow the trapped air to get past. Have built numerous engines for myself, friends air boats and family trucks in 40 years and have always drilled a hole on the T-Stat plate and have never had another issue with air pocket. Now I'm the old timer? Lol.
Hope that helps you or someone else and get your issues worked out.
My phone didn't let me see that post, so I got into it in the middle. Lol.