Rebuild gets hot-any thoughts?

[77chevystepside]

I have started my rebuilt 350 several times and the temperature gauge has to get almost in red before the 180 thermostat opens. I shut it down and then the thermostat opens and the upper hose and radiator gets hot. Have not re-started when it is hot because I worry the engine is overheating. New 16 lb cap and no boil out either and the engine does not really seem that hot. The thermostat, sending unit, & heater core are new too and I know the entire system is full of anti-freeze. Also, I had the radiator flushed and pressure tested prior to install. No leaks anywhere.
Is this common on rebuilds or is it possible I got a bad thermostat or sending unit? I really don't want to pull the thermostat or sending unit this early in the game. The engine did this when the guy who built it started it. We shut it down, re-started it after a few minutes, the temp gauge dropped and the fan pulled a lot of heat through the shroud. He said not to worry but to shut it down if it ever gets in the red before the stat opens. He said this is not unusual on early start ups but I do worry. I will call him tomorrow but just wanting thoughts from others too.

[VileZambonie]

On initial installation Before I install the T-stat, I fill the cooling system through the intake. Then I install the T-stat and top off the radiator. I have an air lift but if you don't have one a spill free funnel is a great thing to have. Run the engine with the funnel on the radiator until the t-stat opens. Shut the engine down, squeeze the upper hose and install the plunger. Release the hose and remove the funnel. Now install your cap and use what's in the funnel to fill your expansion tank.

http://www.sjdiscounttools.com/lis24610.html

[77chevystepside]

Wish I had had that set-up this time. I usually fill the engine through thermostat housing but builder always drills a small hole in the thermostat to allow flow from the upper radiator hose into the engine. This time I filled radiator and then the engine by using a funnel in the upper heater core hose. Due to small hole in thermostat and having the time before start up. I just continued to fill the radiator, squeeze the upper hose and keep it full. Eventually the radiator wouldn't take any more anti-freeze so I thought the heater core might be the only pocket of air. After starting it a few times, the system seemed purged of air but there could be some air in the heater core or engine. Doubt it. I will check it more closley before I start it again. Still working on putting together the body and interior so no rush. Thanks