400 BB build issues

I would not pull the heads again. what do the spark plugs look like, and what part number, try colder. if u use a thermostat, always drill a 1/8" hole in it and test it in a pot of boiling water before install. I do not think a mr gasket therm. is a must, and any therm. is a must, and I always look in the radiator while it is running before it gets too hot to check flow, u should see the coolant flowing well. always have the belt on, and I know u used a box fan, use that and a fan on the water pump. is the ignition timing in the ballpark, is it lean?