Rad Flush - antifreeze question

I know you are focused on trapped air in the system causing the problem and it may be, but on a 1978 van if the radiator is original I would suspect that the radiator flush may have dislodged some stuff causing a blockage in your cooling tubes. You said it ran fine before the flush the only other thing you changed was the thermostat. I have had brand new ones not open all the way. I would put both in a pan of water and bring it to boiling the T stat should open about 1/4 in. Make sure the T stat is installed spring side down I made that mistake once. You can check your radiator with an IR gun and take readings in a grid pattern from top to bottom if you see cold spots that would indicate a blockage. You should read a change in temp from top to bottom of about 50 degrees in a functioning radiator. Also you can shoot the thermostat housing and the sensor in your head that sends the reading to your dash gauge and compare these readings to your IR gun readings. These sensors do fail and gauges can be inaccurate. I would do these simple tests first before you take it in.